# Part kinds > Every kind of part, the key that adds it, what it's for and what usually comes after it. Flozu has 28 kinds of part. A kind says what a part does: how traffic moves through it and which [limits](https://docs.flozu.app/guides/traffic-and-limits.md#limits-on-parts) it can have. It doesn’t decide how the part looks, since any part can show [any drawing](https://docs.flozu.app/guides/looks.md#drawings). A few kinds behave differently from the rest: - **Rate limiters, firewalls, gateways, sign-in and AI models** can have a limit, and turn away what’s past it. - **Queues and topics** hold what’s waiting, and lines into and out of them start as messages. - **Schedulers** start traffic by the clock, so their lines start as messages and a scenario’s busier day doesn’t scale them. - **People, client apps and schedulers** set the pace with the lines they send, so they have no limits of their own. “Usually followed by” is what Flozu suggests when you drop a line from that part on empty canvas. ## People and apps | Kind | Key | What it is | Usually followed by | | ---------- | --- | ----------------------------------------------------------------------------- | -------------------------------------------------- | | Person | `P` | Someone who uses the system, such as a customer, an admin or a support agent. | Client app, CDN, Gateway | | Client app | `C` | An app people use: a website, a mobile app, a desktop app or a device. | Gateway, CDN, DNS, Sign-in, Load balancer, Service | ## The way in | Kind | Key | What it is | Usually followed by | | ------------- | --- | ------------------------------------------------------------------------------ | ------------------------------------------------------- | | DNS | `U` | Turns names like shop.example.com into the addresses traffic is sent to. | CDN, Firewall, Load balancer, Gateway | | CDN | `E` | Serves content from close to people and absorbs traffic before it reaches you. | Gateway, Firewall, Load balancer, Storage, Service | | Firewall | `V` | Lets allowed traffic through and blocks the rest, such as attacks and bots. | Gateway, Load balancer, Service | | Gateway | `G` | The front door for requests. It checks who's calling and routes each request. | Service, Sign-in, Rate limiter, Load balancer, Function | | Sign-in | `I` | Checks who people are and hands out the tokens other parts trust. | Database, Cache, Service | | Load balancer | `L` | Shares traffic between copies of a service so none of them gets swamped. | Service, Function, Gateway | | Rate limiter | `R` | Lets a set amount through each second and turns the rest away. | Service, Gateway, Function | ## Compute | Kind | Key | What it is | Usually followed by | | --------- | --- | ------------------------------------------------------------------------------ | ---------------------------------------------------- | | Service | `S` | A running program that answers requests, such as an API or a web server. | Database, Cache, Queue, Topic, Search, Service | | Worker | `W` | Works through jobs in the background, usually taking them from a queue. | Database, Storage, Service, AI model, Data warehouse | | Function | `F` | Code that runs when it's called and scales up and down by itself. | Database, Storage, Queue, AI model | | Scheduler | `J` | Starts jobs on a timetable, such as a nightly export or a reminder every hour. | Worker, Queue, Function, Service | | AI model | `A` | A language or machine learning model that the system calls. | Service, Database, Storage | ## Messaging | Kind | Key | What it is | Usually followed by | | ----- | --- | ----------------------------------------------------------------------------------- | ------------------------------------------------ | | Queue | `Q` | Holds work in line until something is free to take it. Each item goes to one taker. | Worker, Function, Service | | Topic | `T` | Broadcasts each event to every part that subscribes. | Worker, Function, Service, Queue, Data warehouse | ## Data | Kind | Key | What it is | Usually followed by | | -------------- | --- | ------------------------------------------------------------------------------ | ------------------------------------------------ | | Database | `D` | Keeps the system's records, such as orders, customers and stock. | Database, Topic, Search, Data warehouse, Service | | Cache | `K` | Keeps popular data in memory so it can be read quickly. | Database, Service | | Search | `H` | An index that finds records by their words, such as products in a shop. | Database, Service | | Storage | `B` | Holds files such as images, exports and backups. | Function, CDN, Worker, Data warehouse | | Data warehouse | `O` | Gathers data from across the system so people can report on it and analyse it. | Client app, AI model, Storage | ## Inside a service | Kind | Key | What it is | Usually followed by | | --------- | ---- | -------------------------------------------------------------------------------------------------------- | -------------------------------------------- | | Endpoint | `Y` | A way into a service that others call, such as an HTTP route, an RPC method or an event handler. | Module, Service, Code file, Function | | Module | `M` | A part of a service's code with one job, such as a package or a folder. | Module, Code file, Table, Library, Interface | | Code file | None | One source file, named by its path. | Code file, Interface, Library, Table | | Interface | None | A contract code depends on rather than a thing that runs, such as a TypeScript interface or an API spec. | Code file, Module | | Library | None | A package the code is built on, such as stripe-node or a Go module. | Library | | Table | None | A table, collection or index inside a store, where a service keeps its records. | Table | ## Anything else | Kind | Key | What it is | Usually followed by | | ----------- | --- | ------------------------------------------------------------------ | ------------------------ | | Custom part | `X` | Anything that doesn't fit another kind. It behaves like a service. | Service, Database, Queue |