Firebase applications work even if your app loses its network connection temporarily. We provide several tools for monitoring presence and synchronizing local state with server state, which are introduced in this document.
Managing Presence
In realtime applications it is often useful to detect when clients connect and disconnect. For example, you may want to mark a user as 'offline' when their client disconnects.
Firebase Database clients provide simple primitives that you can use to write to the database when a client disconnects from the Firebase Database servers. These updates occur whether the client disconnects cleanly or not, so you can rely on them to clean up data even if a connection is dropped or a client crashes. All write operations, including setting, updating, and removing, can be performed upon a disconnection.
Here is a simple example of writing data upon disconnection by using the onDisconnect primitive:
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import{getDatabase,ref,onDisconnect}from"firebase/database";constdb=getDatabase();constpresenceRef=ref(db,"disconnectmessage");// Write a string when this client loses connectiononDisconnect(presenceRef).set("I disconnected!");
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varpresenceRef=firebase.database().ref("disconnectmessage");// Write a string when this client loses connectionpresenceRef.onDisconnect().set("I disconnected!");
How onDisconnect Works
When you establish an onDisconnect() operation, the operation lives on the Firebase Realtime Database server. The server checks security to make sure the user can perform the write event requested, and informs the your app if it is invalid. The server then monitors the connection. If at any point the connection times out, or is actively closed by the Realtime Database client, the server checks security a second time (to make sure the operation is still valid) and then invokes the event.
Your app can use the callback on the write operation to ensure the onDisconnect was correctly attached:
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onDisconnect(presenceRef).remove().catch((err)=>{if(err){console.error("could not establish onDisconnect event",err);}});
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presenceRef.onDisconnect().remove((err)=>{if(err){console.error("could not establish onDisconnect event",err);}});
An onDisconnect event can also be canceled by calling .cancel():
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constonDisconnectRef=onDisconnect(presenceRef);onDisconnectRef.set("I disconnected");// some time later when we change our mindsonDisconnectRef.cancel();
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varonDisconnectRef=presenceRef.onDisconnect();onDisconnectRef.set("I disconnected");// some time later when we change our mindsonDisconnectRef.cancel();
Detecting Connection State
For many presence-related features, it is useful for your app to know when it is online or offline. Firebase Realtime Database provides a special location at /.info/connected which is updated every time the Firebase Realtime Database client's connection state changes. Here is an example:
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import{getDatabase,ref,onValue}from"firebase/database";constdb=getDatabase();constconnectedRef=ref(db,".info/connected");onValue(connectedRef,(snap)=>{if(snap.val()===true){console.log("connected");}else{console.log("not connected");}});
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varconnectedRef=firebase.database().ref(".info/connected");connectedRef.on("value",(snap)=>{if(snap.val()===true){console.log("connected");}else{console.log("not connected");}});
/.info/connected is a boolean value which is not synchronized between Realtime Database clients because the value is dependent on the state of the client. In other words, if one client reads /.info/connected as false, this is no guarantee that a separate client will also read false.
Handling Latency
Server Timestamps
The Firebase Realtime Database servers provide a mechanism to insert timestamps generated on the server as data. This feature, combined with onDisconnect, provides an easy way to reliably make note of the time at which a Realtime Database client disconnected:
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import{getDatabase,ref,onDisconnect,serverTimestamp}from"firebase/database";constdb=getDatabase();constuserLastOnlineRef=ref(db,"users/joe/lastOnline");onDisconnect(userLastOnlineRef).set(serverTimestamp());
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varuserLastOnlineRef=firebase.database().ref("users/joe/lastOnline");userLastOnlineRef.onDisconnect().set(firebase.database.ServerValue.TIMESTAMP);
Clock Skew
While firebase.database.ServerValue.TIMESTAMP is much more accurate, and preferable for most read/write operations, it can occasionally be useful to estimate the client's clock skew with respect to the Firebase Realtime Database's servers. You can attach a callback to the location /.info/serverTimeOffset to obtain the value, in milliseconds, that Firebase Realtime Database clients add to the local reported time (epoch time in milliseconds) to estimate the server time. Note that this offset's accuracy can be affected by networking latency, and so is useful primarily for discovering large (> 1 second) discrepancies in clock time.
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import{getDatabase,ref,onValue}from"firebase/database";constdb=getDatabase();constoffsetRef=ref(db,".info/serverTimeOffset");onValue(offsetRef,(snap)=>{constoffset=snap.val();constestimatedServerTimeMs=newDate().getTime()+offset;});
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varoffsetRef=firebase.database().ref(".info/serverTimeOffset");offsetRef.on("value",(snap)=>{varoffset=snap.val();varestimatedServerTimeMs=newDate().getTime()+offset;});
Sample Presence App
By combining disconnect operations with connection state monitoring and server timestamps, you can build a user presence system. In this system, each user stores data at a database location to indicate whether or not a Realtime Database client is online. Clients set this location to true when they come online and a timestamp when they disconnect. This timestamp indicates the last time the given user was online.
Note that your app should queue the disconnect operations before a user is marked online, to avoid any race conditions in the event that the client's network connection is lost before both commands can be sent to the server.
Here is a simple user presence system:
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import{getDatabase,ref,onValue,push,onDisconnect,set,serverTimestamp}from"firebase/database";// Since I can connect from multiple devices or browser tabs, we store each connection instance separately// any time that connectionsRef's value is null (i.e. has no children) I am offlineconstdb=getDatabase();constmyConnectionsRef=ref(db,'users/joe/connections');// stores the timestamp of my last disconnect (the last time I was seen online)constlastOnlineRef=ref(db,'users/joe/lastOnline');constconnectedRef=ref(db,'.info/connected');onValue(connectedRef,(snap)=>{if(snap.val()===true){// We're connected (or reconnected)! Do anything here that should happen only if online (or on reconnect)constcon=push(myConnectionsRef);// When I disconnect, remove this deviceonDisconnect(con).remove();// Add this device to my connections list// this value could contain info about the device or a timestamp tooset(con,true);// When I disconnect, update the last time I was seen onlineonDisconnect(lastOnlineRef).set(serverTimestamp());}});
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// Since I can connect from multiple devices or browser tabs, we store each connection instance separately// any time that connectionsRef's value is null (i.e. has no children) I am offlinevarmyConnectionsRef=firebase.database().ref('users/joe/connections');// stores the timestamp of my last disconnect (the last time I was seen online)varlastOnlineRef=firebase.database().ref('users/joe/lastOnline');varconnectedRef=firebase.database().ref('.info/connected');connectedRef.on('value',(snap)=>{if(snap.val()===true){// We're connected (or reconnected)! Do anything here that should happen only if online (or on reconnect)varcon=myConnectionsRef.push();// When I disconnect, remove this devicecon.onDisconnect().remove();// Add this device to my connections list// this value could contain info about the device or a timestamp toocon.set(true);// When I disconnect, update the last time I was seen onlinelastOnlineRef.onDisconnect().set(firebase.database.ServerValue.TIMESTAMP);}});
