Module types

StreamingFast Substreams module types

Module types overview

Substreams uses two types of modules, map and store.

  • map modules are functions receiving bytes as input and output. These bytes are encoded protobuf messages.

  • store modules are stateful, saving and tracking data through the use of key-value stores.

store modules

store modules write to key-value stores.

Note: To ensure successful and proper parallelization can occur, store modules are not permitted to read any of their own data or values.

Stores declaring their own data types expose methods capable of mutating keys within the store.

Core principle usage of stores

  • Do not save keys in stores unless they are going to be read by a downstream module. Substreams stores are a way to aggregate data, but they are not meant to be a storage layer.

  • Do not save all transfers of a chain in a store module, rather, output them in a map and have a downstream system store them for querying.

There are limitations impose on store usage. Specifically, each key/value entry must be smaller than 10MiB while a store cannot exceed 1GiB total. Keys being string, each character in the key account for 1 byte of storage space.

Important store properties

The two important store properties are valueType,and updatePolicy.

valueType property

The valueType property instructs the Substreams runtime of the data to be saved in the stores.



A basic list of bytes


A UTF-8 string


Decode bytes by using the protobuf definition fully.qualified.Object


A string-serialized integer by using int64 arithmetic operations


A string-serialized floating point value, used for float64 arithmetic operations


A string-serialized integer, supporting precision of any depth


A string-serialized floating point value, supporting precision up to 100 digits

updatePolicy property

The updatePolicy property determines what methods are available in the runtime.

The updatePolicy also defines the merging strategy for identical keys found in two contiguous stores produced through parallel processing.

MethodSupported Value TypesMerge strategy*


bytes, string, proto:...

The last key wins


bytes, string, proto:...

The first key wins


int64, bigint, bigfloat, float64

Values are summed up


int64, bigint, bigfloat, float64

The lowest value is kept


int64, bigint, bigfloat, float64

The highest value is kept


string, bytes

Both keys are concatenated in order. Appended values are limited to 8Kb. Aggregation pattern examples are available in the file

Tip: All update policies provide the delete_prefix method.

The merge strategy is applied during parallel processing.

  • A module has built two partial stores containing keys for segment A, blocks 0-1000, and segment B, blocks 1000-2000, and is prepared to merge them into a complete store.

  • The complete store is represented acting as if the processing was done in a linear fashion, starting at block 0 and proceeding up to block 2000.

Important_: _ To preserve the parallelization capabilities of the system, Substreams is not permitted to read what it has written or read from a store actively being written.

A downstream module is created to read from a store by using one of its inputs to point to the output of the store module.


Ordinals allow a key-value store to have multiple versions of a key within a single block. The store APIs contain different methods of ordinal or ord.

For example, the price for a token can change after transaction B and transaction D, and a downstream module might want to know the value of a key before transaction B and between B and D.

Important: Ordinals must be set every time a key is set and you can only set keys in increasing ordinal order, or by using an ordinal equal to the previous.

In situations where a single key for a block is required and ordering in the store is not important, the ordinal uses a value of zero.

store modes

You can consume data in one of two modes when declaring a store as an input to a module.

get mode

The get mode function provides the module with a key-value store that is guaranteed to be synchronized up to the block being processed. It's possible to query stores by using the get_at, get_last and get_first methods.

Tip: Lookups are local, in-memory, and extremely high-speed.

The definition of store method behavior is:

  • The get_last method is the fastest because it queries the store directly.

  • The get_first method first goes through the current block's deltas in reverse order, before querying the store, in case the key being queried was mutated in the block.

  • The get_at method unwinds deltas up to a specific ordinal, ensuring values for keys set midway through a block are still reachable.

deltas mode

deltas mode provides the module with all the changes occurring in the source store module. Updates, creates, and deletes of the keys mutated during the block processing become available.

Note: When a store is set as an input to the module, it is read-only and you cannot modify, update or mutate them.

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