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global_constants.rst
Global Constants
================
The size limit for :doc:`Michelson <michelson>` contracts is quite small, limited to 60
kilobytes as of Granada protocol. Global constants are a feature added
in Hangzhou protocol that enables the re-use of user-defined Micheline chunks in Michelson scripts, allowing
for larger and more complex contracts on the chain. It works in the
following way:
- Fragments of Michelson code (written in the :doc:`Micheline format <../shell/micheline>`) are
registered on the chain via a new operation
``register_global_constant``. An example expression might be the
integer ``999`` or the lambda expression ``{ PUSH int 999; ADD }``
- Included in the receipt of the operation is a hash of the expression
registered. For example the hash ``999`` is
``expruQN5r2umbZVHy6WynYM8f71F8zS4AERz9bugF8UkPBEqrHLuU8``.
- Constants can be referenced inside a Michelson script with the new
primitive ``constant``. For example, we could write a lambda
equivalent to the one above like so:
``{ PUSH int (constant "expruQN5r2umbZVHy6WynYM8f71F8zS4AERz9bugF8UkPBEqrHLuU8"); ADD }``
Global Constant Registration
----------------------------
The new ``register_global_constant`` operation includes an object with a
single key ``"value"``, the value of which is the Micheline expression
to be registered.
You can submit this operation conveniently through a new :doc:`octez-client <cli-commands>` command.
For example, the command:
.. code:: sh
octez-client register global constant "999" from bootstrap1 --burn-cap 0.017
would result in the output:
::
Node is bootstrapped.
Estimated gas: 1440 units (will add 100 for safety)
Estimated storage: 68 bytes added (will add 20 for safety)
Operation successfully injected in the node.
Operation hash is 'onsFknW5iWa6eiTYqAghY4peQZ7JYQUJg5fR8MwAQkMKjXfNqGf'
NOT waiting for the operation to be included.
Use command
octez-client wait for onsFknW5iWa6eiTYqAghY4peQZ7JYQUJg5fR8MwAQkMKjXfNqGf to be included --confirmations 5 --branch BLockGenesisGenesisGenesisGenesisGenesisCCCCCeZiLHU
and/or an external block explorer to make sure that it has been included.
This sequence of operations was run:
Manager signed operations:
From: tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx
Fee to the baker: ꜩ0.000385
Expected counter: 1
Gas limit: 1540
Storage limit: 88 bytes
Balance updates:
tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx ................ -ꜩ0.000385
fees(the baker who will include this operation,0) ... +ꜩ0.000385
Register Global:
Value: 999
This global constant registration was successfully applied
Balance updates:
tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx ... -ꜩ0.017
Consumed gas: 1440
Storage size: 68 bytes
Global address: expruQN5r2umbZVHy6WynYM8f71F8zS4AERz9bugF8UkPBEqrHLuU8
As you can see, the address of the constant is returned in the operation
receipt in the field ``Global address``. This address is the Base58-encode Blake2b
hash of the binary serialization of the registered Micheline expression.
This means constants are content-addressable - given a particular Micheline
expression, you can always calculate its on-chain address and check if it’s registered.
A few points about registering global constants:
- Global constants may contain references to other constants; however,
any referenced constants must already be registered on the chain. As a
corollary, you cannot have cyclic references.
- Global constants are not type-checked before registration - any
valid Micheline expression may be registered. That said, attempting
to originate a contract that uses a constant in an ill-typed way will
fail.
- The total depth of the expression registered as a constant (after
expanding all constant references) may not exceed 10,000.
- The total number of nodes in the Micheline expression being
registered (after expanding all constant references) may not exceed
the ``max_micheline_node_count`` protocol constant. As of Hangzhou
this is 50,000.
- The total number of bytes in the Micheline expression being
registered (after expanding all constant references) may not exceed
the ``max_micheline_bytes_limit`` protocol constant. As of Hangzhou
this is 50,000.
Originating a Contract that uses Global Constants
-------------------------------------------------
A global constant can be referenced in Michelson scripts via the
primitive ``constant``, which accepts a single string argument, being
the hash of the expression to be referenced at runtime. This primitive
can be used to replace any Micheline node in the bodies of the
``parameter``, ``storage``, ``code``, or ``view`` fields of a Michelson script. For
example, we replace every instance of the type ``lambda unit unit`` and
value 999 with their respective hashes:
.. code-block:: michelson
parameter (constant "exprtYirrFwYKm6yKLzJNtYRbq49zedYq16BonRvMzHiwSbUekB9YL");
storage (big_map int (constant "exprtYirrFwYKm6yKLzJNtYRbq49zedYq16BonRvMzHiwSbUekB9YL"));
code {
PUSH int (constant "expruQN5r2umbZVHy6WynYM8f71F8zS4AERz9bugF8UkPBEqrHLuU8");
# <rest of code>
};
The full expansion of this contract would be:
.. code-block:: michelson
parameter (lambda unit unit);
storage (big_map int (lambda unit unit));
code {
PUSH int 999;
# <rest of code>
};
During origination, all constants are expanded recursively. The
operation will fail if the resulting contract is ill-typed. Global
constant expansion consumes gas; thus, the operation may also fail due
to gas exhaustion.
Global Constants at Runtime
---------------------------
Contracts that use global constants are semantically equivalent to the
contract with all constants expanded.
Note that using the `UNPACK <https://tezos.gitlab.io/michelson-reference/#instr-UNPACK>`__
operation to deserialize a lambda which contains a reference to a global
constant is not supported. Similarly, originating a contract which contains
a reference to a global constant using the
`CREATE_CONTRACT <https://tezos.gitlab.io/michelson-reference/#instr-CREATE_CONTRACT>`__
instruction will also fail.
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