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# Algorand Specifications

When an [_application call_](https://specs.algorand.co/ledger/ledger-txn-application-call) transaction is evaluated,
it is processed according to the following procedure.

The transaction effects **MUST NOT** be visible to other transactions until the
points marked **SUCCEED** below.

**FAIL** indicates that any modifications to state up to that point **MUST** be
discarded and the entire transaction rejected.

- If the [_application ID_](https://specs.algorand.co/ledger/ledger-txn-application-call#application-id) specified
by the transaction is zero, create a new application with ID equal to one plus the
system transaction counter (this is the same ID selection algorithm as used by [Assets](https://specs.algorand.co/ledger/ledger-txn-semantics-asset#asset-configuration)).

When creating an application, the application parameters specified by the transaction
(Approval Program, Clear State Program, Global State Schema, Local State Schema,
and Extra Program Pages) are allocated into the sender’s account data, keyed
by the new application ID.

Continue to [Step 2](https://specs.algorand.co/ledger/ledger-txn-semantics-application#step-2).

- If the [_application ID_](https://specs.algorand.co/ledger/ledger-txn-application-call#application-id) specified
by the transaction is nonzero, continue to [Step 2](https://specs.algorand.co/ledger/ledger-txn-semantics-application#step-2).

- If the [_on completion action_](https://specs.algorand.co/ledger/ledger-txn-application-call#on-completion-action)
is equal to `ClearStateOC`, then:
  - Check if the transaction’s sender is opted in to this [_application ID_](https://specs.algorand.co/ledger/ledger-txn-application-call#application-id).
    If not, **FAIL**.

- Check if the application parameters still exist in the creator’s account data.
    - If the application does not exist, delete the sender’s local state for this
      application (marking them as no longer opted in), and **SUCCEED**.

- If the application does exist, continue to [Step 3](https://specs.algorand.co/ledger/ledger-txn-semantics-application#step-3).
- If the [_on completion action_](https://specs.algorand.co/ledger/ledger-txn-application-call#on-completion-action)
is not equal to `ClearStateOC`, continue to [Step 4](https://specs.algorand.co/ledger/ledger-txn-semantics-application#step-4).

- Execute the Clear State Program.
  - If the program execution returns `PASS == true`, apply the local/global key/value
    store deltas generated by the program’s execution.

- If the program execution returns `PASS == false`, do not apply any local/global
    key/value store deltas generated by the program’s execution.

- Delete the sender’s local state for this application (marking them as no longer
    opted in). **SUCCEED**.

- If [_on completion action_](https://specs.algorand.co/ledger/ledger-txn-application-call#on-completion-action)
is equal to `OptInOC`, then at this point during execution we will allocate a local
key/value store for the sender for this application ID, marking the sender as opted
in.

Continue to [Step 5](https://specs.algorand.co/ledger/ledger-txn-semantics-application#step-5).

- Execute the Approval Program.
  - If the program execution returns `PASS == true`, apply any local/global key/value
    store deltas generated by the program’s execution. Continue to [Step 6](https://specs.algorand.co/ledger/ledger-txn-semantics-application#step-6).

- If the program execution returns `PASS == false`, **FAIL**.

- If [_on completion action_](https://specs.algorand.co/ledger/ledger-txn-application-call#on-completion-action)
is equal to `NoOpOC`
  - **SUCCEED**.
- If [_on completion action_](https://specs.algorand.co/ledger/ledger-txn-application-call#on-completion-action)
is equal to`OptInOC`
  - This was handled above. **SUCCEED**.
- If [_on completion action_](https://specs.algorand.co/ledger/ledger-txn-application-call#on-completion-action)
is equal to `CloseOutOC`
  - Check if the transaction’s sender is opted in to this application ID. If not,
    **FAIL**.

- Delete the sender’s local state for this application (marking them as no longer
    opted in). **SUCCEED.**
- If [_on completion action_](https://specs.algorand.co/ledger/ledger-txn-application-call#on-completion-action)
is equal to `ClearStateOC`
  - This was handled above (unreachable).
- If [_on completion action_](https://specs.algorand.co/ledger/ledger-txn-application-call#on-completion-action)
is equal to `DeleteApplicationOC`
  - Delete the application’s parameters from the creator’s account data. (Note:
    this does not affect any local state). **SUCCEED**.
- If [_on completion action_](https://specs.algorand.co/ledger/ledger-txn-application-call#on-completion-action)
is equal to `UpdateApplicationOC`
  - If an existing program is version 4 or higher, and the supplied program is a
downgrade from the existing version. **FAIL**.

- Update the Approval Program and Clear State Program for this application according
to the programs specified in this _application call_ transaction and increment
the Application Version. The new programs are not executed in this transaction.
    **SUCCEED**.

- Before the execution of the first _application call_ transaction in a _group_,
the combined size of all boxes referred to in the box references of all transactions
in the group **MUST NOT** exceed the I/O budget (i.e., BoxIOBoxIO
times the total number of box references in the group), or else the group fails.

- During the execution of an Approval Program or Clear State Program, the application’s
Local State Schema and Global State Schema **SHALL** never be violated. The program’s
execution will fail on the first instruction that would cause the relevant schema
to be violated. Writing a `Bytes` value to a local or global [Key/Value Store](https://specs.algorand.co/ledger/ledger-applications#keyvalue-stores)
such that the sum of the lengths of the key and value in bytes exceeds \MaxAppBytesValueLen\MaxAppBytesValueLen,
or writing any value to a key longer than \MaxAppKeyLen\MaxAppKeyLen bytes, will likewise
cause the program to fail on the offending instruction.

- During the execution of an Approval Program, the total size of all boxes that are
created or modified in the group **MUST NOT** exceed the I/O budget. The program’s
execution will fail on the first instruction that would cause the constraint to be
violated. If a box is deleted after creation or modification, its size is not considered
in this sum.

- An attempt to access resources that are _not available_ will cause the program
execution to fail.

- Boxes **MAY NOT** be accessed by an application’s Clear State Program.

- In addition to the group’s named Boxes, transactions **MAY** also access the Boxes
of Applications that were previously created in the same group. Across the execution
of the entire group, the group can access as many of these unnamed boxes as the group
has empty box references.

> For further details on AVM Resource Availability, refer to the [AVM Specifications](https://specs.algorand.co/avm/avm-mode-applications#resource-availability).