NAMECACHE — DHT caching of GNS results — GNUnet documentation

# NAMECACHE — DHT caching of GNS results

The NAMECACHE subsystem is responsible for caching (encrypted) resolution results of the GNU Name System (GNS). GNS makes zone information available to other users via the DHT. However, as accessing the DHT for every lookup is expensive (and as the DHT's local cache is lost whenever the peer is restarted), GNS uses the NAMECACHE as a more persistent cache for DHT lookups. Thus, instead of always looking up every name in the DHT, GNS first checks if the result is already available locally in the NAMECACHE. Only if there is no result in the NAMECACHE, GNS queries the DHT. The NAMECACHE stores data in the same (encrypted) format as the DHT. It thus makes no sense to iterate over all items in the NAMECACHE – the NAMECACHE does not have a way to provide the keys required to decrypt the entries.

Blocks in the NAMECACHE share the same expiration mechanism as blocks in the DHT – the block expires wheneever any of the records in the (encrypted) block expires. The expiration time of the block is the only information stored in plaintext. The NAMECACHE service internally performs all of the required work to expire blocks, clients do not have to worry about this. Also, given that NAMECACHE stores only GNS blocks that local users requested, there is no configuration option to limit the size of the NAMECACHE. It is assumed to be always small enough (a few MB) to fit on the drive.

The NAMECACHE supports the use of different database backends via a plugin API.

## libgnunetnamecache

The NAMECACHE API consists of five simple functions. First, there is `GNUNET_NAMECACHE_connect` to connect to the NAMECACHE service. This returns the handle required for all other operations on the NAMECACHE. Using `GNUNET_NAMECACHE_block_cache` clients can insert a block into the cache. `GNUNET_NAMECACHE_lookup_block` can be used to lookup blocks that were stored in the NAMECACHE. Both operations can be canceled using `GNUNET_NAMECACHE_cancel`. Note that canceling a `GNUNET_NAMECACHE_block_cache` operation can result in the block being stored in the NAMECACHE --- or not. Cancellation primarily ensures that the continuation function with the result of the operation will no longer be invoked. Finally, `GNUNET_NAMECACHE_disconnect` closes the connection to the NAMECACHE.

The maximum size of a block that can be stored in the NAMECACHE is `GNUNET_NAMECACHE_MAX_VALUE_SIZE`, which is defined to be 63 kB.

## The NAMECACHE Client-Service Protocol

All messages in the NAMECACHE IPC protocol start with the `struct GNUNET_NAMECACHE_Header` which adds a request ID (32-bit integer) to the standard message header. The request ID is used to match requests with the respective responses from the NAMECACHE, as they are allowed to happen out-of-order.

### Lookup

The `struct LookupBlockMessage` is used to lookup a block stored in the cache. It contains the query hash. The NAMECACHE always responds with a `struct LookupBlockResponseMessage`. If the NAMECACHE has no response, it sets the expiration time in the response to zero. Otherwise, the response is expected to contain the expiration time, the ECDSA signature, the derived key and the (variable-size) encrypted data of the block.

### Store

The `struct BlockCacheMessage` is used to cache a block in the NAMECACHE. It has the same structure as the `struct LookupBlockResponseMessage`. The service responds with a `struct BlockCacheResponseMessage` which contains the result of the operation (success or failure). In the future, we might want to make it possible to provide an error message as well.

## The NAMECACHE Plugin API

The NAMECACHE plugin API consists of two functions, `cache_block` to store a block in the database, and `lookup_block` to lookup a block in the database.

### Lookup2

The `lookup_block` function is expected to return at most one block to the iterator, and return `GNUNET_NO` if there were no non-expired results. If there are multiple non-expired results in the cache, the lookup is supposed to return the result with the largest expiration time.

### Store2

The `cache_block` function is expected to try to store the block in the database, and return `GNUNET_SYSERR` if this was not possible for any reason. Furthermore, `cache_block` is expected to implicitly perform cache maintenance and purge blocks from the cache that have expired. Note that `cache_block` might encounter the case where the database already has another block stored under the same key. In this case, the plugin must ensure that the block with the larger expiration time is preserved. Obviously, this can done either by simply adding new blocks and selecting for the most recent expiration time during lookup, or by checking which block is more recent during the store operation.