initial versionHEADmaster

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diff --git a/.gitignore b/.gitignore
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+.idea/
+.code/
+.vscode/
+
+run/
\ No newline at end of file
diff --git a/LICENSE b/LICENSE
new file mode 100644
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--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,674 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
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+OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
+IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
+ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. Limitation of Liability.
+
+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+    <program>  Copyright (C) <year>  <name of author>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<https://www.gnu.org/licenses/>.
+
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.  But first, please read
+<https://www.gnu.org/licenses/why-not-lgpl.html>.
diff --git a/cmd/cmd.go b/cmd/cmd.go
new file mode 100644
index 0000000..8374124
--- /dev/null
+++ b/cmd/cmd.go
@@ -0,0 +1,101 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"github.com/akamensky/argparse"
+	"github.com/bombsimon/logrusr/v4"
+	"github.com/go-logr/logr"
+	"github.com/hit-mc/observatory/config"
+	"github.com/hit-mc/observatory/observatory"
+	"github.com/sirupsen/logrus"
+	"net/http"
+	"os"
+	"os/signal"
+	"time"
+)
+
+func main() {
+	parser := argparse.NewParser("observatory", "Minecraft tunnel observatory")
+	configFile := parser.String("c", "config", &argparse.Options{
+		Required: true,
+		Help:     "Config file to use",
+		Default:  "config.toml",
+	})
+	isServer := parser.Flag("", "server", &argparse.Options{
+		Required: false,
+		Help:     "Run program as server (stats collector)",
+		Default:  false,
+	})
+	isClient := parser.Flag("", "client", &argparse.Options{
+		Required: false,
+		Help:     "Run program as client (stats observer&reporter)",
+		Default:  false,
+	})
+	err := parser.Parse(os.Args)
+	if err != nil {
+		fmt.Print(parser.Usage(err))
+		return
+	}
+	if !*isServer && !*isClient {
+		fmt.Println("You must specify either --server or --client in CLI arguments.")
+		os.Exit(1)
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	logger := logrusr.New(logrus.New())
+	go func() {
+		logger := logger.WithName("SignalHandler")
+		c := make(chan os.Signal, 1)
+		signal.Notify(c, os.Interrupt)
+		<-c
+		logger.Info("stopping")
+		cancel()
+	}()
+
+	if *isServer {
+		runCollector(ctx, *configFile, logger)
+	} else {
+		runObserver(ctx, *configFile, logger)
+	}
+}
+
+func runCollector(ctx context.Context, configFile string, logger logr.Logger) {
+	cfg, err := config.Read[config.Server](configFile)
+	if err != nil {
+		panic(fmt.Errorf("error reading config file `%v`: %w", configFile, err))
+	}
+
+	handshakeTimeout := time.Duration(cfg.HandshakeTimeout) * time.Millisecond
+	s := observatory.NewCollector(cfg.Listen, handshakeTimeout, logger, cfg.Token, cfg.Targets)
+	err = s.Run(ctx)
+	if err != nil && !errors.Is(err, http.ErrServerClosed) {
+		panic(err)
+	}
+}
+
+func runObserver(ctx context.Context, configFile string, logger logr.Logger) {
+	cfg, err := config.Read[config.Client](configFile)
+	if err != nil {
+		panic(fmt.Errorf("error reading config file `%v`: %w", configFile, err))
+	}
+
+	sendBuf := cfg.SendBuffer
+	if sendBuf < 0 {
+		sendBuf = 0
+	}
+
+	reconnInterval := cfg.ReconnectInterval
+	if reconnInterval < 0 {
+		reconnInterval = 0
+	}
+
+	reportConnTimeout := cfg.ReportConnectTimeout
+	if reportConnTimeout < 0 {
+		reportConnTimeout = 0
+	}
+
+	observatory.RunClient(ctx, cfg, logger)
+}
diff --git a/config/client.go b/config/client.go
new file mode 100644
index 0000000..a2d1500
--- /dev/null
+++ b/config/client.go
@@ -0,0 +1,43 @@
+package config
+
+import (
+	"fmt"
+	"github.com/pelletier/go-toml/v2"
+	"os"
+)
+
+type Server struct {
+	Listen           string   `toml:"listen"`
+	HandshakeTimeout int      `toml:"handshake_timeout"`
+	Token            string   `toml:"token"`
+	Targets          []Target `toml:"targets"`
+}
+
+type Client struct {
+	ObserverID           string `toml:"observer_id"`
+	ReportServer         string `toml:"report_server"`
+	CheckInterval        int    `toml:"check_interval"`
+	SendBuffer           int    `toml:"send_buffer"`
+	ReconnectInterval    int    `toml:"reconnect_interval"`
+	ReportConnectTimeout int    `toml:"report_connect_timeout"`
+	Token                string `toml:"token"`
+}
+
+type Target struct {
+	Host string `toml:"host"`
+	Port int    `toml:"port"`
+}
+
+func Read[T any](path string) (ret *T, err error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("open: %w", err)
+	}
+	defer func() {
+		_ = f.Close()
+	}()
+	dec := toml.NewDecoder(f)
+	ret = new(T)
+	err = dec.Decode(ret)
+	return ret, err
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..edf841f
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,18 @@
+module github.com/hit-mc/observatory
+
+go 1.18
+
+require (
+	github.com/akamensky/argparse v1.4.0
+	github.com/bombsimon/logrusr/v4 v4.1.0
+	github.com/go-logr/logr v1.4.1
+	github.com/gorilla/websocket v1.5.1
+	github.com/pelletier/go-toml/v2 v2.1.1
+	github.com/sirupsen/logrus v1.9.0
+	github.com/xrjr/mcutils v1.5.1
+)
+
+require (
+	golang.org/x/net v0.17.0 // indirect
+	golang.org/x/sys v0.13.0 // indirect
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..835c3dd
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,36 @@
+github.com/akamensky/argparse v1.4.0 h1:YGzvsTqCvbEZhL8zZu2AiA5nq805NZh75JNj4ajn1xc=
+github.com/akamensky/argparse v1.4.0/go.mod h1:S5kwC7IuDcEr5VeXtGPRVZ5o/FdhcMlQz4IZQuw64xA=
+github.com/bombsimon/logrusr/v4 v4.1.0 h1:uZNPbwusB0eUXlO8hIUwStE6Lr5bLN6IgYgG+75kuh4=
+github.com/bombsimon/logrusr/v4 v4.1.0/go.mod h1:pjfHC5e59CvjTBIU3V3sGhFWFAnsnhOR03TRc6im0l8=
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/go-logr/logr v1.4.1 h1:pKouT5E8xu9zeFC39JXRDukb6JFQPXM5p5I91188VAQ=
+github.com/go-logr/logr v1.4.1/go.mod h1:9T104GzyrTigFIr8wt5mBrctHMim0Nb2HLGrmQ40KvY=
+github.com/gorilla/websocket v1.5.1 h1:gmztn0JnHVt9JZquRuzLw3g4wouNVzKL15iLr/zn/QY=
+github.com/gorilla/websocket v1.5.1/go.mod h1:x3kM2JMyaluk02fnUJpQuwD2dCS5NDG2ZHL0uE0tcaY=
+github.com/pelletier/go-toml/v2 v2.1.1 h1:LWAJwfNvjQZCFIDKWYQaM62NcYeYViCmWIwmOStowAI=
+github.com/pelletier/go-toml/v2 v2.1.1/go.mod h1:tJU2Z3ZkXwnxa4DPO899bsyIoywizdUvyaeZurnPPDc=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/sirupsen/logrus v1.9.0 h1:trlNQbNUG3OdDrDil03MCb1H2o9nJ1x4/5LYw7byDE0=
+github.com/sirupsen/logrus v1.9.0/go.mod h1:naHLuLoDiP4jHNo9R0sCBMtWGeIprob74mVsIT4qYEQ=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/objx v0.4.0/go.mod h1:YvHI0jy2hoMjB+UWwv71VJQ9isScKT/TqJzVSSt89Yw=
+github.com/stretchr/objx v0.5.0/go.mod h1:Yh+to48EsGEfYuaHDzXPcE3xhTkx73EhmCGUpEOglKo=
+github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+github.com/stretchr/testify v1.8.0/go.mod h1:yNjHg4UonilssWZ8iaSj1OCr/vHnekPRkoO+kdMU+MU=
+github.com/stretchr/testify v1.8.4 h1:CcVxjf3Q8PM0mHUKJCdn+eZZtm5yQwehR5yeSVQQcUk=
+github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
+github.com/xrjr/mcutils v1.5.1 h1:E4ScafEH+joxWVgRCGpJBJfxWX/vaNJDcoS40V91rVo=
+github.com/xrjr/mcutils v1.5.1/go.mod h1:43n8cyMIHYjiRM2LFZLVH5Ppz2+RvWppz6OgkLP8Lsk=
+golang.org/x/net v0.17.0 h1:pVaXccu2ozPjCXewfr1S7xza/zcXTity9cCdXQYSjIM=
+golang.org/x/net v0.17.0/go.mod h1:NxSsAGuq816PNPmqtQdLE42eU2Fs7NoRIZrHJAlaCOE=
+golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.13.0 h1:Af8nKPmuFypiUBjVoU9V20FiaFXOcuZI21p0ycVYYGE=
+golang.org/x/sys v0.13.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/observatory/client.go b/observatory/client.go
new file mode 100644
index 0000000..fc4d2b6
--- /dev/null
+++ b/observatory/client.go
@@ -0,0 +1,266 @@
+package observatory
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"github.com/go-logr/logr"
+	"github.com/gorilla/websocket"
+	"github.com/hit-mc/observatory/config"
+	"github.com/hit-mc/observatory/protocol"
+	"github.com/xrjr/mcutils/pkg/ping"
+	"net/http"
+	"slices"
+	"sync"
+	"time"
+)
+
+func RunClient(ctx context.Context, cfg *config.Client, logger logr.Logger) {
+
+	// [Communicator] -> [intermediate goroutine] -> [Pinger]
+	configUpdates := make(chan *protocol.ServerPushInfo, 8)
+	// [Pinger] -> [Communicator]
+	pingResults := make(chan *protocol.Observation, cfg.SendBuffer)
+
+	pinger := Pinger{
+		Logger:   logger.WithName("pinger"),
+		Output:   pingResults,
+		Interval: time.Duration(cfg.CheckInterval) * time.Millisecond,
+	}
+	comm := Communicator{
+		ServerAddr:        cfg.ReportServer,
+		ConnectTimeout:    time.Duration(cfg.ReportConnectTimeout) * time.Millisecond,
+		ReconnectInterval: time.Duration(cfg.ReconnectInterval) * time.Millisecond,
+		Token:             cfg.Token,
+		ObserverID:        cfg.ObserverID,
+		Logger:            logger.WithName("communicator"),
+		ConfigUpdate:      configUpdates,
+		StateReport:       pingResults,
+	}
+
+	wg := &sync.WaitGroup{}
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		defer close(configUpdates)
+		comm.Run(ctx)
+	}()
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for cu := range configUpdates {
+			// apply config updates to pinger
+			pinger.UpdateTargets(cu.Targets)
+		}
+	}()
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		defer close(pingResults)
+		pinger.Run(ctx)
+	}()
+
+	wg.Wait()
+}
+
+type Observer struct {
+	serverAddr           string
+	interval             time.Duration
+	observations         chan *protocol.Observation
+	logger               logr.Logger
+	reconnectInterval    time.Duration
+	reportConnectTimeout time.Duration
+	token                string
+	id                   string
+}
+
+func readServerPushInfo(c *websocket.Conn) (*protocol.ServerPushInfo, error) {
+	for {
+		typ, data, err := c.ReadMessage()
+		if err != nil {
+			return nil, fmt.Errorf("read websocket message: %w", err)
+		}
+		if typ != websocket.BinaryMessage {
+			continue
+		}
+		var info protocol.ServerPushInfo
+		err = json.Unmarshal(data, &info)
+		if err != nil {
+			return nil, fmt.Errorf("unmarshal ServerPushInfo: %w", err)
+		}
+		if info.Version != protocol.CurrentServerPushInfoVersion {
+			return nil, fmt.Errorf("invalid ServerPushInfo version `%v`, raw data: `%v`",
+				info.Version, string(data))
+		}
+		return &info, nil
+	}
+}
+
+type Pinger struct {
+	targets  []protocol.Target
+	mu       sync.RWMutex
+	Logger   logr.Logger
+	Output   chan<- *protocol.Observation
+	Interval time.Duration
+}
+
+func (p *Pinger) Run(ctx context.Context) {
+	tk := time.NewTicker(p.Interval)
+	defer tk.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-tk.C:
+			p.ping()
+		}
+	}
+}
+
+func (p *Pinger) ping() {
+	var targets []protocol.Target
+	func() {
+		p.mu.RLock()
+		defer p.mu.RUnlock()
+		targets = slices.Clone(p.targets)
+	}()
+	var wg sync.WaitGroup
+	defer wg.Wait()
+	for i := range targets {
+		wg.Add(1)
+		i := i
+		go func() {
+			defer wg.Done()
+			p.Output <- ping0(targets[i], p.Logger)
+		}()
+	}
+}
+
+func ping0(target protocol.Target, log logr.Logger) *protocol.Observation {
+	t := time.Now()
+	_, latency, err := ping.Ping(target.Host, int(target.Port))
+	if err != nil {
+		log.Error(err, "ping failed", "target", target.String())
+		return &protocol.Observation{
+			Time:   protocol.TimeStamp(t),
+			Target: target,
+			Online: false,
+		}
+	}
+	return &protocol.Observation{
+		Time:    protocol.TimeStamp(t),
+		Target:  target,
+		Online:  true,
+		Latency: uint32(latency),
+	}
+}
+
+func (p *Pinger) UpdateTargets(targets []protocol.Target) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	p.targets = slices.Clone(targets)
+}
+
+type Communicator struct {
+	ServerAddr        string
+	ConnectTimeout    time.Duration
+	ReconnectInterval time.Duration
+	Token             string
+	ObserverID        string
+	Logger            logr.Logger
+	ConfigUpdate      chan<- *protocol.ServerPushInfo
+	StateReport       <-chan *protocol.Observation
+	sendBuffer        []*protocol.Observation // states that are taken out but not sent
+}
+
+// Run blocks until StateReport or ctx is closed
+func (c *Communicator) Run(ctx context.Context) {
+	for {
+		if ctx.Err() != nil {
+			return
+		}
+		err := c.run(ctx)
+		if err != nil {
+			if ctx.Err() != nil {
+				return
+			}
+			c.Logger.Error(err, "waiting for reconnect")
+			sleep(ctx, c.ReconnectInterval)
+			continue
+		}
+		return
+	}
+}
+
+func (c *Communicator) run(ctx context.Context) error {
+	dialer := websocket.Dialer{
+		Proxy:            http.ProxyFromEnvironment,
+		HandshakeTimeout: c.ConnectTimeout,
+	}
+	headers := http.Header{
+		protocol.TokenHeader:      []string{c.Token},
+		protocol.ObserverIDHeader: []string{c.ObserverID},
+	}
+	conn, _, err := dialer.Dial(c.ServerAddr, headers)
+	if err != nil {
+		return fmt.Errorf("dial websocket: %w", err)
+	}
+	defer func() {
+		_ = conn.Close()
+	}()
+
+	serverConfig, err := readServerPushInfo(conn)
+	if err != nil {
+		return fmt.Errorf("error reading server side config: %w", err)
+	}
+	c.Logger.Info("received observe targets from server", "targets", serverConfig.Targets)
+
+	c.ConfigUpdate <- serverConfig
+
+	sendData := func(ob *protocol.Observation) error {
+		data, err := json.Marshal(*ob)
+		if err != nil {
+			c.Logger.Error(err, "error marshalling observation result, drop")
+			return nil
+		}
+		return conn.WriteMessage(websocket.BinaryMessage, data)
+	}
+
+	for len(c.sendBuffer) > 0 {
+		err := sendData(c.sendBuffer[0])
+		if err != nil {
+			c.Logger.Error(err, "error writing websocket message, try reconnect")
+			return err
+		}
+		c.sendBuffer = c.sendBuffer[1:]
+	}
+
+	for {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case ob, ok := <-c.StateReport:
+			if !ok {
+				return nil
+			}
+			err := sendData(ob)
+			if err != nil {
+				c.Logger.Error(err, "error writing websocket message, try reconnect")
+				c.sendBuffer = append(c.sendBuffer, ob) // store unsent messages to queue
+				return err
+			}
+		}
+	}
+}
+
+func sleep(ctx context.Context, d time.Duration) {
+	tm := time.NewTimer(d)
+	defer tm.Stop()
+	select {
+	case <-ctx.Done():
+	case <-tm.C:
+	}
+}
diff --git a/observatory/server.go b/observatory/server.go
new file mode 100644
index 0000000..ea338f0
--- /dev/null
+++ b/observatory/server.go
@@ -0,0 +1,261 @@
+package observatory
+
+import (
+	"cmp"
+	"context"
+	"encoding/json"
+	"fmt"
+	"github.com/go-logr/logr"
+	"github.com/gorilla/websocket"
+	"github.com/hit-mc/observatory/config"
+	"github.com/hit-mc/observatory/protocol"
+	"io"
+	"net"
+	"net/http"
+	"slices"
+	"sync"
+	"time"
+)
+
+func NewCollector(
+	listen string,
+	handshakeTimeout time.Duration,
+	logger logr.Logger,
+	token string,
+	targets []config.Target,
+) *Collector {
+	targets2 := make([]protocol.Target, len(targets))
+	for i := range targets {
+		targets2[i] = protocol.Target{
+			Host: targets[i].Host,
+			Port: uint16(targets[i].Port),
+		}
+	}
+	return &Collector{
+		listen:           listen,
+		handshakeTimeout: handshakeTimeout,
+		logger:           logger,
+		stats:            newServerStatus(),
+		token:            token,
+		targets:          targets2,
+	}
+}
+
+type Collector struct {
+	listen           string
+	handshakeTimeout time.Duration
+	logger           logr.Logger
+	stats            serverStatus
+	token            string
+	targets          []protocol.Target
+}
+
+func (c *Collector) Run(ctx context.Context) error {
+	ssc, err := json.Marshal(protocol.ServerPushInfo{
+		Version: protocol.CurrentServerPushInfoVersion,
+		Targets: c.targets,
+	})
+	if err != nil {
+		return fmt.Errorf("error marshalling ServerPushInfo: %w", err)
+	}
+
+	up := websocket.Upgrader{
+		HandshakeTimeout: c.handshakeTimeout,
+	}
+	mux := http.NewServeMux()
+	mux.HandleFunc("/stats", func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != "GET" {
+			w.WriteHeader(405)
+			return
+		}
+		stats := c.stats.GetStats()
+		data, err := json.Marshal(stats)
+		if err != nil {
+			w.WriteHeader(500)
+			_, _ = io.WriteString(w, fmt.Sprintf("error marshalling JSON: %v", err))
+			return
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write(data)
+	})
+	mux.HandleFunc("/collect", func(w http.ResponseWriter, r *http.Request) {
+		conn, err := up.Upgrade(w, r, nil)
+		if err != nil {
+			c.logger.Error(err, "error upgrading connection protocol to websocket")
+			return
+		}
+		defer func() {
+			_ = conn.Close()
+		}()
+		token := r.Header.Get(protocol.TokenHeader)
+		if token != c.token {
+			c.logger.Error(nil, "reject connection with invalid token", "token", token)
+			return
+		}
+		observerID := r.Header.Get(protocol.ObserverIDHeader)
+		if observerID == "" {
+			return
+		}
+		c.logger.Info("observer connected successfully",
+			"endpoint", r.RemoteAddr, "observer_id", observerID)
+
+		const pingInterval = 10 * time.Second
+		const maxPingWait = 20 * time.Second
+		conn.SetPongHandler(func(string) error {
+			_ = conn.SetReadDeadline(time.Now().Add(pingInterval + maxPingWait))
+			return nil
+		})
+
+		writeMu := &sync.Mutex{}
+		writeMessage := func(typ int, data []byte) error {
+			writeMu.Lock()
+			defer writeMu.Unlock()
+			return conn.WriteMessage(websocket.BinaryMessage, ssc)
+		}
+
+		wg := &sync.WaitGroup{}
+		// send keepalive packets periodically
+		wg.Add(1)
+		go func() {
+			for {
+				err := writeMessage(websocket.PingMessage, nil)
+				if err != nil {
+					c.logger.Error(err, "error pinging client, disconnect",
+						"observer_id", observerID)
+					_ = conn.Close()
+					return
+				}
+				sleep(r.Context(), pingInterval)
+			}
+		}()
+		defer wg.Wait()
+
+		// send server-side config
+		err = writeMessage(websocket.BinaryMessage, ssc)
+		if err != nil {
+			c.logger.Error(err, "error writing server-side-config")
+			return
+		}
+
+		for {
+			typ, data, err := conn.ReadMessage()
+			if err != nil {
+				c.logger.Error(err, "error reading websocket message")
+				return
+			}
+			if typ != websocket.BinaryMessage {
+				continue
+			}
+			var observation protocol.Observation
+			err = json.Unmarshal(data, &observation)
+			if err != nil {
+				c.logger.Error(err, "error unmarshalling message from client")
+				return
+			}
+			c.stats.Put(&observation, observerID)
+		}
+	})
+	server := http.Server{
+		Addr:              c.listen,
+		Handler:           mux,
+		ReadTimeout:       10 * time.Second,
+		ReadHeaderTimeout: 10 * time.Second,
+		WriteTimeout:      10 * time.Second,
+		IdleTimeout:       300 * time.Second,
+		BaseContext: func(net.Listener) context.Context {
+			return ctx
+		},
+	}
+	go func() {
+		<-ctx.Done()
+		_ = server.Close()
+	}()
+	err = server.ListenAndServe()
+	if err != nil {
+		return fmt.Errorf("error starting HTTP server: %w", err)
+	}
+	return nil
+}
+
+func newServerStatus() serverStatus {
+	return serverStatus{
+		status: make(map[protocol.Target][]protocol.SourcedObservation),
+	}
+}
+
+type serverStatus struct {
+	mu     sync.RWMutex
+	status map[protocol.Target][]protocol.SourcedObservation
+}
+
+func (s *serverStatus) Put(observation *protocol.Observation, source string) {
+	entry := protocol.SourcedObservation{
+		Observation: *observation,
+		Source:      source,
+	}
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	// replace existing entry with the same observer
+	for i, ob := range s.status[observation.Target] {
+		if ob.Source == source {
+			s.status[observation.Target][i] = entry
+			goto done
+		}
+	}
+	// add new entry if not exist
+	s.status[observation.Target] = append(s.status[observation.Target], entry)
+	slices.SortFunc(s.status[observation.Target], func(a, b protocol.SourcedObservation) int {
+		return cmp.Compare(a.Source, b.Source)
+	})
+done:
+	s.purge()
+}
+
+// purge removes state entries lived longer than 180s
+// Note: caller should hold write lock
+func (s *serverStatus) purge() {
+	const ttl = 180 * time.Second
+	t0 := time.Now().Add(-ttl)
+	for target := range s.status {
+		ss := s.status[target]
+		purged := false
+		for i := range ss {
+			if time.Time(ss[i].Time).Before(t0) {
+				ss[i].Time = protocol.TimeStamp{}
+				purged = true
+			}
+		}
+		if purged {
+			s.status[target] = shrink(ss)
+		}
+	}
+}
+
+func shrink(s []protocol.SourcedObservation) []protocol.SourcedObservation {
+	i := 0 // slow pointer
+	j := 0 // fast pointer
+	for j < len(s) {
+		if !time.Time(s[j].Time).IsZero() {
+			i++
+		}
+		j++
+		if j < len(s) {
+			s[i] = s[j]
+		}
+	}
+	if i != j {
+		return s[:i]
+	}
+	return s
+}
+
+func (s *serverStatus) GetStats() protocol.TargetStats {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	ret := make(protocol.TargetStats, len(s.status))
+	for t, os := range s.status {
+		ret[t.String()] = os
+	}
+	return ret
+}
diff --git a/observatory/server_test.go b/observatory/server_test.go
new file mode 100644
index 0000000..453da7a
--- /dev/null
+++ b/observatory/server_test.go
@@ -0,0 +1,33 @@
+package observatory
+
+import (
+	"github.com/hit-mc/observatory/protocol"
+	"math/rand"
+	"testing"
+	"time"
+)
+
+func Fuzz_shrink(f *testing.F) {
+	f.Fuzz(func(t *testing.T, seed int64) {
+		s := rand.NewSource(seed)
+		n := s.Int63() % 1023
+		ss := make([]protocol.SourcedObservation, n)
+		nonzero := protocol.TimeStamp(time.Unix(10000, 10000))
+		n2 := 0
+		for i := int64(0); i < n; i++ {
+			if s.Int63()%17 < 12 {
+				ss[i].Time = nonzero
+				n2++
+			}
+		}
+		ss2 := shrink(ss)
+		if len(ss2) != n2 {
+			t.Fatal("invalid length ", len(ss2), " expected", n2)
+		}
+		for i := range ss2 {
+			if time.Time(ss2[i].Time).IsZero() {
+				t.Fatal("invalid time at ", i)
+			}
+		}
+	})
+}
diff --git a/protocol/handshake.go b/protocol/handshake.go
new file mode 100644
index 0000000..02e3f0d
--- /dev/null
+++ b/protocol/handshake.go
@@ -0,0 +1,14 @@
+package protocol
+
+const (
+	TokenHeader      = "X-Observatory-Token"
+	ObserverIDHeader = "X-Observatory-Observer-ID"
+)
+
+// ServerPushInfo is the first server->client packet, applying configs to observers
+type ServerPushInfo struct {
+	Version uint64   `json:"server_push_info_version"`
+	Targets []Target `json:"targets"`
+}
+
+const CurrentServerPushInfoVersion uint64 = 240308
diff --git a/protocol/messages.go b/protocol/messages.go
new file mode 100644
index 0000000..d8f5697
--- /dev/null
+++ b/protocol/messages.go
@@ -0,0 +1,29 @@
+package protocol
+
+import (
+	"encoding/json"
+	"time"
+)
+
+type TimeStamp time.Time
+
+func (t *TimeStamp) UnmarshalJSON(data []byte) error {
+	var ts int64
+	err := json.Unmarshal(data, &ts)
+	if err != nil {
+		return err
+	}
+	*t = TimeStamp(time.UnixMilli(ts))
+	return nil
+}
+
+func (t TimeStamp) MarshalJSON() ([]byte, error) {
+	return json.Marshal(time.Time(t).UnixMilli())
+}
+
+type Observation struct {
+	Time    TimeStamp `json:"time"`
+	Target  Target    `json:"target"`
+	Online  bool      `json:"online"`
+	Latency uint32    `json:"latency"`
+}
diff --git a/protocol/observe.go b/protocol/observe.go
new file mode 100644
index 0000000..48d052a
--- /dev/null
+++ b/protocol/observe.go
@@ -0,0 +1,14 @@
+package protocol
+
+import (
+	"fmt"
+)
+
+type Target struct {
+	Host string `json:"host"`
+	Port uint16 `json:"port"`
+}
+
+func (t Target) String() string {
+	return fmt.Sprintf("%s:%d", t.Host, t.Port)
+}
diff --git a/protocol/publish.go b/protocol/publish.go
new file mode 100644
index 0000000..13e6ee1
--- /dev/null
+++ b/protocol/publish.go
@@ -0,0 +1,8 @@
+package protocol
+
+type TargetStats map[string][]SourcedObservation
+
+type SourcedObservation struct {
+	Observation
+	Source string `json:"source,omitempty"`
+}