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79 lines
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<h1>Restoration of Synapse after losing 1 year of database history</h1>
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<div class="article"><h2 id="introduction">Introduction</h2><p>In July 2025, my Synapse PostgreSQL had broken, so I needed to restore it from my 1-y.o. backup. I was thinking about what to do, and my friend sent me <a href="https://blog.erethon.com/blog/2023/06/21/what-happens-when-a-matrix-server-disappears/">this</a> article, so we understood what to do!</p><p>For example, there is a room called #example:example.org (roomid !example:example.org), whose member was @user:example.net.
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The problem is that every server knows @user:example.net is in the room #example:example.org. But our server doesn’t! So that is what we should aim at solving the problem.</p><h2 id="architecture">Architecture</h2><p>We get 2 types of error messages:</p><ul>
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<li><code>synapse.handlers.receipts: [PUT-...] Ignoring receipt for room '!example:example.org' from server example.org as we're not in the room</code> (there is the same error message, but when you get a typing event from an unknown room)</li>
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<li><code>synapse.federation.federation_server: [PUT-...] Ignoring PDU for unknown room_id: !example:example.org</code></li>
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</ul><p>We can read the journal and parse the error message (I use a bash script to do that).
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Okay, we have a room id! In the given article, the author is the only user of their server, so they can just send <a href="https://spec.matrix.org/v1.15/client-server-api/#post_matrixclientv3joinroomidoralias">POST /_matrix/client/v3/join/!example:example.org</a> request, and everything will be good.
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But my server has multiple users, so I can’t use the easiest way. We need to get information about membership (so we need to get state).</p><p>As we are admins of our servers, we have their signing keys.
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We will need them because we are going to get state via Federation API requests.
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How? There are different ways to do that.
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However, all of them require at least 1 event id, so firstly we need to do <a href="https://spec.matrix.org/v1.15/server-server-api/#get_matrixfederationv1timestamp_to_eventroomid">GET /_matrix/federation/v1/timestamp_to_event</a>.
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Determining request parameters (?dir as direction (b[ackward] or f[orward]) and &ts as timestamp in Unix time format in milliseconds), we have just got either the last or the first event id! In my case, I chose the way of getting the last event, so then I’ll have to make <a href="https://spec.matrix.org/v1.15/server-server-api/#get_matrixfederationv1stateroomid">GET /_matrix/federation/v1/state/!example:example.org?event_id=$event_id</a>, and I will get the actual state.
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Of course, both of the requests, as I said, should be signed using your server’s key (<a href="https://spec.matrix.org/v1.15/server-server-api/#signing-events">how to sign events</a>).</p><br/><p>(There is no support of timestamp_to_event in Conduit, so it would be a good idea to make support of requesting different servers)</p><p>After we receive the state, we need to parse it, but I think this can’t make a difficulty.
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As a result, we have a list of our users that were in the given room, like [“@user:example.net”, “@anotheruser:example.net”, “@exampleuser:example.net”].
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Then, the simplest way is to get temporary user’s auth token via <a href="https://github.com/matrix-org/synapse/blob/develop/docs/admin_api/user_admin_api.md#login-as-a-user">POST /_synapse/admin/v1/users/user_id/login</a> and make <a href="https://spec.matrix.org/v1.15/client-server-api/#post_matrixclientv3joinroomidoralias">POST /_matrix/client/v3/join/!example:example.org</a> from the name of the user using their token.
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The problem is solved!</p><h2 id="external-links">External links</h2><ul>
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<li><a href="https://blog.erethon.com/blog/2023/06/21/what-happens-when-a-matrix-server-disappears/">Article that gave me the idea</a></li>
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<li><a href="https://spec.matrix.org/v1.15/server-server-api/">Matrix Federation API docs</a>
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<ul>
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<li><a href="https://spec.matrix.org/v1.15/server-server-api/#signing-events">How to sign Federation API requests</a></li>
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<li><a href="https://spec.matrix.org/v1.15/server-server-api/#get_matrixfederationv1stateroomid">GET /_matrix/federation/v1/state/!example:example.org?event_id=$event_id</a></li>
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<li><a href="https://spec.matrix.org/v1.15/server-server-api/#get_matrixfederationv1timestamp_to_eventroomid">GET /_matrix/federation/v1/timestamp_to_event</a></li>
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</ul>
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</li>
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<li><a href="https://spec.matrix.org/v1.15/server-server-api/">Matrix Client API docs</a>
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<ul>
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<li><a href="https://spec.matrix.org/v1.15/client-server-api/#post_matrixclientv3joinroomidoralias">POST /_matrix/client/v3/join/!example:example.org</a></li>
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</ul>
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</li>
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<li><a href="https://github.com/matrix-org/synapse/blob/develop/docs/admin_api/user_admin_api.md#login-as-a-user">Admin API POST /_synapse/admin/v1/users/user_id/login</a></li>
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</ul></div>
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<main>
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<pre> .--.
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_.-" ,;;|
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_.-=" _," ,,;;;'
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.-"`_.-"``-..,,;;;;:'
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`"'` `\`\
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/^\\\
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</pre>
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