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Schema

Lightning Stream can sync LMDBs in two different modes:

  1. If the application is Lightning Stream aware and uses native Lightning Stream headers for all its values, it can directly use these headers to sync and merge data. This is described in the native schema section.
  2. If the application does not use the special Lightning Stream headers, data can still be synced, but Lightning Stream needs to create shadow DBIs to keep track of all the sync metadata. This is described in the non-native schema (shadow) section.

General schema considerations

No matter if you are using the native mode or non-native mode, there are some important considerations regarding how the application manages its data, that affect if it is safe to use Lightning Stream with multiple active writers.

Conceptually, consider an LMDB with Lightning Stream like a global key-value storage that is updated by multiple instances at the same time. Does the way these values are stored by the application allow that to be done safely?

Application caching of state

If the application assumes that it is the only one writing to the LMDB, it may cache certain state and not be aware of changes made by Lightning Stream when syncing data from other instances.

For example, it could have an in-memory cache of the state and only invalidate this cache when itself writes a change to the LMDB.

Another example would be where the LMDB would map IDs to Names, and the application maintains an in-memory reverse index from Name to ID.

Solution

Since LMDB is so fast, it may be feasible to store all state in the LMDB and read it on demand.

Alternatively, a cache could be short-lived (e.g. 1 second), or the application could check if the LMDB's LastTxnID has changed since the last cache update.

!!! TODO

Link to a section describing LMDB concepts like LastTxnID.

Natural keys vs. Sequential IDs

If an application uses sequential IDs as keys, using multiple writers will quickly result in a conflict, because it is very likely that two instances will try to create a new entry using the same ID.

Natural keys, on the other hand, do not have this problem. For example, if an entry describes a domain name, use the domain name itself as the key, instead of a number, if possible. Even if two instances try to create the same entry, it will not result in an inconsistent database, as the natural key automatically prevents the addition of duplicate domain entries. A hash of the fields that make up the uniqueness constraint provides similar guarantees.

If natural keys cannot be used, use random or globally unique IDs like UUIDs to reduce the chance of an ID clash. The larger the ID, the smaller the chance of a clash. In this case you do need to be aware that duplicate entries can occur, for example if two users try to add an example.com entry on different instances at the same time: you will end up with two example.com entries with different IDs.

Multi-value entries and indices

Suppose you have a DBI that keeps track of tags or categories for keys. One way to organise this would be:

  • "key 1" => "RED,BLUE"
  • "key 2" => "RED,GREEN,YELLOW"

Now consider what happens if two instances want to update the tags for "key 1" at the same time with an additional tag:

  • Instance A: "key 1" => "RED,BLUE,YELLOW"
  • Instance B: "key 1" => "RED,BLUE,CYAN"

One of these updates will win the race, and the other one will get lost.

For these tags this may be acceptable, but consider a scenario where these are not tags, but indices:

  • "accounts-owned-by-jane" => "14,522,1314"

If there is no process to automatically fix these indices, this can cause serious issues.

Solution

Use multiple entries, with the individual IDs encoded in the key.

Example:

  • "accounts-owned-by-jane:14" => ""
  • "accounts-owned-by-jane:522" => ""
  • "accounts-owned-by-jane:1314" => ""

The value part can be kept empty, or contain some data, as needed.

When you need a list of values, perform an MDB_SET_RANGE query to fetch all keys that start with "accounts-owned-by-jane:".

If a safe separator cannot be found, the key can be prefixed with one or two bytes indicating the length of the key. Also keep in mind that LMDB keys are typically limited to 511 bytes.

These records can safely be added and deleted by different instances of the application.

!!! warning

You may be tempted to solve this with `MDB_DUPSORT`, but Lightning Stream only supports dupsort
DBIs in non-native mode, and then only with [severe caveats](schema-shadow.md#the-dupsort_hack).