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Serialization

Between contract calls, the code of a contract, as well as its storage, are stored as a serialized sequence of bytes, for efficiency purposes. Every time the contract is called, the serialized code and storage are deserialized, unless the deserialized version is still cached. Similarly, after the execution of the contract, the storage needs to be serialized to be stored again as a sequence of bytes.

This takes CPU time, which means that when you call an entrypoint, on top of paying for the gas for the execution of the code of the entrypoint itself, you also need to pay for this serialization/deserialization. The cost to call a very simple entrypoint may get large if there is a lot of data in its storage.

Remember that unlike the rest of the storage, big-maps are not entirely serialized/deserialized for each call. Instead, only the values that are read are deserialized, and only the values that are added or updated are serialized. This makes using big-maps more efficient in these cases.

PACK and UNPACK​

Tezos provides the ability to serialize and deserialize data or code yourself:

  • The PACK instruction takes a value of (almost) any type, and serializes it into a bytes value.
  • The UNPACK instruction takes a bytes value, and deserializes it into its original value. As the deserialization may be impossible if the sequence of bytes doesn't represent valid serialized data, it returns an option type.

Serializing your own data in this way may be useful if you want to apply operations that are only available on bytes values.

For example, you may want to compute the hash of some data. You can do so by packing it first and then applying a hash function such as BLAKE2B on the resulting bytes value.

Implementation details​

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