# Chapter 10: Who builds the conversation list > Ana logs in to the desktop app on a new laptop for the first time. What should the chat list on the left show? Does each device build the list from the messages it receives, or does the server build it and devices sync it? IM Systems in Depth · https://im.liko.page/en/fan-out/ The first thing v2 adds is a desktop app. Ana buys a new laptop and logs in for the first time. What should the chat list on the left show? She has joined a lot of groups: **2,000 conversations** (the series assumes 200 on average). Many are quiet. Nobody has said anything in the family chat for over a month, or in her classmates’ group for almost five. Where those conversations come from depends on **who builds the chat list**. Chapter 6 already keeps a row per person per conversation on the server, and a new device that copies it gets them all. This chapter makes that the rule, and explains why not the other way: letting each device build its own list from the messages. ## 1. Two ways to build it *[Interactive figure: open the page to use it, https://im.liko.page/en/fan-out/]* - **On the device**: the server gives each person a timeline, an inbox, with one small entry for every message sent to them, in time order (which conversation, which seq: 32 bytes). The phone groups the entries by conversation, and that is its chat list. - **On the server**: the server keeps a conversation record per person per conversation, and the phone copies them. ## 2. Tied to fan-out **Building on the device needs write fan-out.** A phone can only learn which conversations it has from the messages it receives, so every message has to add an entry to every recipient’s timeline. That is write fan-out. The hiking group has 500 members, so one message adds 500 entries; 1,000 messages a day add 500,000. A timeline only grows, and has to be trimmed. **Building on the server lets a message be stored once.** The list lives on the server and the message only in its conversation (read fan-out); a message in the hiking group writes nothing for its members. Whether to keep a per-person stream as well is the read/write trade-off chapter 6 worked out, and it chose not to. Telegram builds on the server and keeps a stream too: the chat list comes from the server ([`messages.getDialogs`](https://core.telegram.org/method/messages.getDialogs)), and private chats and basic groups also share one [update sequence](https://core.telegram.org/api/updates) per user. ## 3. A new laptop’s first login Built on the device, the new laptop can only **replay the timeline**: download the inbox and group it by conversation. How much it gets back depends on how long the inbox is kept: - **Kept briefly** (say 7 days, to save storage): a group that has been quiet for months has no entry in the timeline, so it does not appear. - **Kept forever**: everything comes back, but the whole history has to come down first. Ana receives 267 messages a day; a year of entries is about 3.1 MB, and it grows the longer she uses the app. Built on the server, the new laptop **copies the conversations**: 2,000 rows, the same size after one year or five. In short: **a timeline grows with messages; the conversations grow only with how many there are**. A new device wants the conversations, and copying them is easier than replaying. *[Interactive figure: open the page to use it, https://im.liko.page/en/fan-out/]* With a 7-day inbox the new laptop finds only 380 conversations; the family chat and the classmates’ group are both missing. Built on the server, all 2,000 are there. Settings on a conversation, such as pinned and muted, sync separately; this chapter does not compare them. ## 4. The decision for v2 **The chat list follows the server: the server keeps the conversations (the conversation records), every device copies them, and the local list is only a copy.** A message is still stored once, in its conversation. ## 5. The cost Nothing more is stored; chapter 6 already keeps the records. The new costs are on the device: - **A new device fetches every conversation at first.** A record and a latest seq per conversation (36 bytes), plus the newest message of each (about 200 bytes): about 472 KB for 2,000. It can fetch the first screen first and the rest as the user scrolls. - **The local list drifts from the server’s.** Say a conversation was deleted while offline, or a sync was missed. The rule: the server wins, and the device puts itself back in line. What is still open is chapter 6’s bill: **every sync asks every conversation**, 24 KB each time for Ana. That is the next chapter. ## 6. Other answers - **An inbox, built on the device.** The server is simple, and syncing is one stream and one round trip. The cost is section 3: a short inbox loses conversations, a long one means replaying a lot of history, and settings need a sync of their own. - **Built on the server, plus a per-person stream.** Like Telegram: private chats and small groups get “everything since last time” in one go. There are two syncs to maintain, and a group that grows has to switch from one to the other. ## 7. This chapter’s decision *[Interactive figure: open the page to use it, https://im.liko.page/en/fan-out/]* **Decision card** - Problem: v2 adds a desktop app. Ana logs in on a new laptop: where does the chat list come from? She has 2,000 conversations, many silent for months. - Choice: The chat list follows the server: the server keeps the conversations (chapter 6’s conversation records), not a timeline; every device copies them and the local list is only a copy. A message is still stored once. - Cost: A new device fetches every conversation at first (about 472 KB for 2,000; the first screen can come first); the device puts its local list back in line with the server. Still open: every sync asks every conversation. - Revisit when: Getting only the conversations that changed (chapter 11); read and unread (chapter 15); several devices (chapter 16). - Other answers: An inbox, built on the device (a short one loses conversations, a long one means replaying history); built on the server plus a per-person stream (Telegram). With the conversations on the server, every device agrees. But each time Ana opens her phone it still downloads the latest seq of all 2,000, when only a dozen or so have changed. Next: syncing the conversation list.