# Internal c++ interfaces

The following interfaces are defined here:

* [`Chain`](chain.h) — used by wallet to access blockchain and mempool state. Added in [#14437](https://github.com/bitcoin/bitcoin/pull/14437), [#14711](https://github.com/bitcoin/bitcoin/pull/14711), [#15288](https://github.com/bitcoin/bitcoin/pull/15288), and [#10973](https://github.com/bitcoin/bitcoin/pull/10973).

* [`ChainClient`](chain.h) — used by node to start & stop `Chain` clients. Added in [#14437](https://github.com/bitcoin/bitcoin/pull/14437).

* [`Node`](node.h) — used by GUI to start & stop bitcoin node. Added in [#10244](https://github.com/bitcoin/bitcoin/pull/10244).

* [`Wallet`](wallet.h) — used by GUI to access wallets. Added in [#10244](https://github.com/bitcoin/bitcoin/pull/10244).

* [`Handler`](handler.h) — returned by `handleEvent` methods on interfaces above and used to manage lifetimes of event handlers.

* [`Init`](init.h) — used by multiprocess code to access interfaces above on startup. Added in [#19160](https://github.com/bitcoin/bitcoin/pull/19160).

* [`Ipc`](ipc.h) — used by multiprocess code to access `Init` interface across processes. Added in [#19160](https://github.com/bitcoin/bitcoin/pull/19160).

The interfaces above define boundaries between major components of bitcoin code (node, wallet, and gui), making it possible for them to run in [different processes](../../doc/multiprocess.md), and be tested, developed, and understood independently. These interfaces are not currently designed to be stable or to be used externally.