# Metadata and reserved names
# Stream metadata
Every stream in EventStoreDB has metadata stream associated with it, prefixed by $$, so the metadata stream from a stream called foo is $$foo. EventStoreDB allows you to change some values in the metadata, and you can write your own data into stream metadata that you can refer to in your code.
# Reserved names
All internal data used by EventStoreDB is prefixed with a $ character (for example $maxCount on a stream's metadata). Because of this you should not use names with a $ prefix as event names, metadata keys, or stream names, except where detailed below.
The supported internal settings are:
| Property name | Description |
|---|---|
$maxAge | Sets a sliding window based on dates. When data reaches a certain age it disappears automatically from the stream and is considered eligible for scavenging. This value is set as an integer representing the number of seconds. This value must be >= 1. |
$maxCount | Sets a sliding window based on the number of items in the stream. When data reaches a certain length it disappears automatically from the stream and is considered eligible for scavenging. This value is set as an integer representing the count of items. This value must be >= 1. |
$cacheControl | This controls the cache of the head of a stream. Most URIs in a stream are infinitely cacheable but the head by default will not cache. It may be preferable in some situations to set a small amount of caching on the head to allow intermediaries to handle polls (say 10 seconds). The argument is an integer representing the seconds to cache. This value must be >= 1. |
TIP
If you set both $maxAge and $maxCount then events will become eligible for scavenging when either criteria is met. For example, if you set $maxAge to 10 and $maxCount to 50,000, events will be marked as eligible for scavenging after either 10 seconds, or 50,000 events, have passed. Deleted items will only be removed once the scavenge process runs.
Security access control lists are also included in the $acl section of the stream metadata.
| Property name | Description |
|---|---|
$r | The list of users with read permissions |
$w | The list of users with write permissions |
$d | The list of users with delete permissions |
$mw | The list of users with write permissions to stream metadata |
$mr | The list of users with read permissions to stream metadata |
You can find more details on access control lists can here.
# Event metadata
Every event in EventStoreDB has metadata associated with it. EventStoreDB allows you to change some values in the metadata, and you can write your own data into event metadata that you can refer to in your code.
All names starting with $ are reserved space for internal use. The currently supported reserved internal names are:
| Property name | Description |
|---|---|
$correlationId | The application level correlation ID associated with this message. |
$causationId | The application level causation ID associated with this message. |
Projections honor both the correlationId and causationId patterns for any events it produces internally (linkTo/emit/etc).