# System Integrators Guide

tip

This documentation is currently under construction with more being added on a regular basis.

If you provide blockchain services to your customers and wish to add the Aptos blockchain to your platform, then this guide is for you. This system integrators guide will walk you through all you need to integrate the Aptos blockchain into your platform. This guide assumes that you are familiar with the blockchains.

## Overview​

This guide will overview the following topics for integrating with Aptos:

• Preparing an environment for testing.
• Create an account on the blockchain.
• Exchange account identifiers with another entity on the blockchain, for example, to perform swaps.
• Create a transaction.
• Obtain a gas estimate and validate the transaction for correctness.
• Submit the transaction to the blockchain.
• Wait for the outcome of the transaction.
• Query historical transactions and interactions for a given account with a specific account, i.e., withdraws and deposits.

## Getting Started​

There are two well-supported approaches for integrating with the Aptos blockchain:

1. Local development using our standalone testnet
2. Devnet -- a shared resource for the community, data resets weekly, weekly update from aptos-core main branch.
3. Testnet -- a shared resource for the community, data will be preserved, network configuration will mimic Mainnet.

### Local Testnet​

There are two options to run a local testnet:

1. Directly run a local testnet is to follow this guide.
2. Use the CLI by 1) installing with the CLI and 2) start a local node with a faucet

This will expose a REST API service at http://127.0.0.1:8080/v1 and a Faucet service at http://127.0.0.1:8000 for option 1 or http://127.0.0.1:8081 for option 2. The applications will output the location of the services.

### Access Devnet​

Faucet service: https://faucet.devnet.aptoslabs.com REST API service: https://fullnode.devnet.aptoslabs.com/v1

### Access Testnet​

Faucet service: https://faucet.testnet.aptoslabs.com REST API service: https://fullnode.testnet.aptoslabs.com/v1

### SDKs​

Aptos currently provides three SDKs:

## Accounts on Aptos​

An account represents a resource on the Aptos blockchain that can send transactions. Each account is identified by a particular 32-byte account address and is a container for Move modules and Move resources. On Aptos, accounts must be created on-chain prior to any blockchain operations involving that account. The Aptos framework supports implicitly creating accounts when transferring Aptos coin via aptos_account::transfer or explicitly via aptos_account::create_account.

At creation, an Aptos account contains:

• A resource containing Aptos Coin and deposit and withdrawal of coins from that resource.
• An authentication key associated with their current public, private key(s).
• A strictly increasing sequence number that represents the account's next transaction's sequence number to prevent replay attacks.
• A strictly increasing number that represents the next distinct GUID creation number.
• An event stream for all new types of coins added to the account.
• An event stream for all key rotations for the account.

### Account identifiers​

Currently, Aptos only supports a single, unified identifier for an account. Accounts on Aptos are universally represented as a 32-byte hex string. A hex string shorter than 32-bytes is also valid: in those scenarios, the hex string is padded with leading zeroes, e.g., 0x1 => 0x0000000000000...01.

Account addresses are defined at creation time as a one-way function from the public key(s) and signature algorithm used for authentication for the account.

This is covered in depth in the Accounts documentation and demonstrated in the Typescript SDK and Python SDK. Note that currently these SDKs only demonstrate how to generate an address from an Ed25519 single signer.

### Rotating the keys​

An Account on Aptos has the ability to rotate keys so that potentially compromised keys cannot be used to access the accounts. Keys can be rotated via account::rotate_authentication_key function.

Refreshing the keys is generally regarded as good hygiene in the security field. However, this presents a challenge for system integrators who are used to using a mnemonic to represent both a private key and its associated account. To simplify this for the system integrators, Aptos will provide an on-chain mapping, before the launch of the mainnet. The on-chain data maps an effective account address as defined by the current mnemonic to the actual account address.

### Preventing replay attacks​

When the Aptos blockchain processes the transaction, it looks at the sequence number in the transaction and compares it with the sequence number in the sender’s account (as stored on the blockchain at the current ledger version). The transaction is executed only if the sequence number in the transaction is the same as the sequence number for the sender account, and rejects if they do not match. In this way past transactions, which necessarily contain older sequence numbers, cannot be replayed, hence preventing replay attacks.

## Transactions​

Aptos transactions are encoded in BCS (Binary Canonical Serialization). Transactions contain information such as the sender’s account address, authentication from the sender, the desired operation to be performed on the Aptos blockchain, and the amount of gas the sender is willing to pay to execute the transaction.

### Transaction states​

A transaction may end in one of the following states:

1. Committed on the blockchain and executed. This is considered as a successful transaction.
2. Committed on the blockchain and aborted. The abort code indicates why the transaction failed to execute.
3. Discarded during transaction submission due to a validation check such as insufficient gas, invalid transaction format, or incorrect key.
4. Discarded after transaction submission but before attempted execution. This could be due to timeouts or insufficient gas due to other transactions affecting the account.

The sender’s account will be charged gas for any committed transactions.

During transaction submission, the submitter is notified of successful submission or a reason for failing validations otherwise.

A transaction that is successfully submitted but ultimately discarded may have no visible state in any accessible Aptos node or within the Aptos network. A user can attempt to resubmit the same transaction to re-validate the transaction. If the submitting node believes that this transaction is still valid, this will return an error stating that there exists an identical transaction already submitted.

The submitter can try to increase the gas cost by a trivial amount to help make progress and adjust for whatever may have been causing the discarding of the transaction further downstream.

On the Aptos devnet, the time between submission and confirmation is within seconds.

### Constructing a transaction​

Aptos supports two methods for constructing transactions:

• Constructing JSON-encoded objects and interacting with the Web API to generate native transactions.
• Using the Aptos client libraries to generate native transactions.

#### JSON-encoded transactions​

JSON-encoded transactions can be generated via the REST API, following these steps:

• First construct an appropriate JSON payload for the /transactions/encode_submission endpoint as demonstrated in the Python SDK.
• The output of the above contains an object containing a message and this must be signed with the sender’s private key locally.
• Finally, the original JSON payload is extended with the signature information and posted to the /transactions endpoint. This will return back a transaction submission result that, if successful, contains a transaction hash in the hash field.

JSON-encoded transactions allow for rapid development and support seamless ABI conversions of transaction arguments to native types. However, most system integrators prefer to generate transactions within their own tech stack. Both the TypeScript SDK and Python SDK support generating BCS transactions.

#### BCS-encoded transactions​

BCS encoded transactions can be submitted to the /transactions endpoint but must specify Content-Type: application/x.aptos.signed_transaction+bcs in the HTTP headers. This will return back a transaction submission result that, if successful, contains a transaction hash in the hash field.

### Types of Transactions​

Within a given transaction, the target of execution can be one of two types:

• An entry point (formerly known as script function), and/or

Currently the SDKs: Python and Typescript only support the generation of transactions that target entry points. This guide points out many of those entry points, such as coin::transfer and aptos_account::create_account.

All operations on the Aptos blockchain should be available via entry point calls. While one could submit multiple transactions calling entry points in series, many such operations may benefit from being called atomically from a single transaction. A script payload transaction can call any entry point or public function defined within any module.

Move book

Currently there are no tutorials in this guide on script payloads, but the Move book does go in some depth.

See the following documentation for generating valid transactions:

### Status of a transaction​

Transaction status can be obtained by querying the API /transactions/by_hash/{hash} with the hash returned during the submission of the transaction.

A reasonable strategy for submitting transactions is to limit their lifetime to 30 to 60 seconds, and polling that API at regular intervals until success or a several seconds after that time has elapsed. If there is no commitment on-chain, the transaction was likely discarded.

### Testing transactions or transaction pre-execution​

To facilitate evaluation of transactions, Aptos supports a simulation API that does not require and should not contain valid signatures on transactions.

The simulation API works identical to the transaction submission API, except that it executes the transaction and returns back the results along with the gas used. The simulation API can be accessed by submitting a transaction to /transactions/simulate.

Here's an example showing how to use the simulation API in the Typescript SDK. Note that the gas use may change based upon the state of the account. We recommend that the maximum gas amount be larger than the amount quoted by this API.

## Viewing current and historical state​

Most integrations into the Aptos blockchain benefit from a holistic and comprehensive overview of the current and historical state of the blockchain. Aptos provides historical transactions, state, and events, which are the result of transaction execution.

• Historical transactions specify the execution status, output, and tie to related events. Each transaction has a unique version number associated with it that dictates its global sequential ordering in the history of the blockchain ledger.
• The state is the representation of all transaction outputs up to a specific version. In other words, a state version is the accumulation of all transactions inclusive of that transaction version.
• As transactions execute, they may emit events. Events are hints about changes in on-chain data.

The storage service on a node employs two forms of pruning that erase data from nodes:

1. state, and
2. events, transactions, and everything else.

While either of these may be disabled, storing the state versions is not particularly sustainable.

Events and transactions pruning can be disabled via setting the enable_ledger_pruner) to false. This will be default behavior in Mainnet. In the near future, Aptos will provide indexers that mitigate the need to directly query from a node.

The REST API contains the following useful APIs for querying transactions and events:

## Exchanging and tracking coins​

Aptos has a standard Coin type. Different types of coins can be represented in this type through the use of distinct structs that represent the type parameter or generic for Coin<T>.

Coins are stored within an account under the resource CoinStore<T>. At account creation, each user has the resource CoinStore<0x1::aptos_coin::AptosCoin> or CoinStore<AptosCoin>, for short. Within this resource is the Aptos coin: Coin<AptosCoin>.

### Transferring coins between users​

Coins can be transferred between users via the coin::transfer function for all coins and aptos_account::transfer for Aptos coins. The advantage of the latter function is that it creates the destination account if it does not exist.

caution

It is important to note, that if an account has not registered a CoinStore<T> for a given T, then any transfer of type T to that account will fail.

### Current balance for a coin​

The current balance for a Coin<T> where T is the Aptos coin is available at the account resources url: https://{rest_api_server}/accounts/{address}/resource/0x1::coin::CoinStore<0x1::aptos_coin::AptosCoin>. The balance is stored within coin::amount. The resource also contains the total number of deposit and withdraw events, the counter value within deposit_event and withdraw_event, respectively.

{  "type": "0x1::coin::CoinStore<0x1::aptos_coin::AptosCoin>",  "data": {    "coin": {      "value": "3927"    },    "deposit_events": {      "counter": "1",      "guid": {        "id": {          "addr": "0xcb2f940705c44ba110cd3b4f6540c96f2634938bd5f2aabd6946abf12ed88457",          "creation_num": "2"        }      }    },    "withdraw_events": {      "counter": "1",      "guid": {        "id": {          "addr": "0xcb2f940705c44ba110cd3b4f6540c96f2634938bd5f2aabd6946abf12ed88457",          "creation_num": "3"        }      }    }  }}

### Querying transactions​

In Aptos, each transaction is committed as a distinct version to the Blockchain. This allows for the convenience of sharing committed transactions by their version number, to do so query https://{rest_server_api}/transactions/by_version/{version}. Transactions submitted by an account can also be queried via https://{rest_server_api}/account/{address}/transactions?start={sequence_number}&limit=1, where the sequence_number matches the sequence number of the transaction.

A transfer transaction would appear as follows:

{  "version": "13629679",  "gas_used": "4",  "success": true,  "vm_status": "Executed successfully",  "changes": [    {      "address": "0xb258b91eee04111039320a85b0c24a2dd433909e14a6b5c32ee722e0fdecfddc",      "data": {        "type": "0x1::coin::CoinStore<0x1::aptos_coin::AptosCoin>",        "data": {          "coin": {            "value": "1000"          },          "deposit_events": {            "counter": "1",            "guid": {              "id": {                "addr": "0x5098df8e7969b58ab3bd2d440c6203f64c60a1fd5c08b9d4abe6ae4216246c3e",                "creaton_num": "2",              }            }          },          ...        }      },      "type": "write_resource"    },    ...  ],  "sender": "0x810026ca8291dd88b5b30a1d3ca2edd683d33d06c4a7f7c451d96f6d47bc5e8b",  "sequence_number": "0",  "max_gas_amount": "2000",  "gas_unit_price": "1",  "expiration_timestamp_secs": "1660616127",  "payload": {    "function": "0x1::coin::transfer",    "type_arguments": [      "0x1::aptos_coin::AptosCoin"    ],    "arguments": [      "0x5098df8e7969b58ab3bd2d440c6203f64c60a1fd5c08b9d4abe6ae4216246c3e",      "1000"    ],    "type": "entry_function_payload"  },  "events": [    {      "key": "0x0300000000000000810026ca8291dd88b5b30a1d3ca2edd683d33d06c4a7f7c451d96f6d47bc5e8b",      "guid": {        "id": {          "addr": "0x810026ca8291dd88b5b30a1d3ca2edd683d33d06c4a7f7c451d96f6d47bc5e8b",          "creation_num": "3"          }        }      },      "sequence_number": "0",      "type": "0x1::coin::WithdrawEvent",      "data": {        "amount": "1000"      }    },    {      "key": "0x02000000000000005098df8e7969b58ab3bd2d440c6203f64c60a1fd5c08b9d4abe6ae4216246c3e",      guid": {        "id": {          "addr": "0x5098df8e7969b58ab3bd2d440c6203f64c60a1fd5c08b9d4abe6ae4216246c3e",          "creation_num": "2"          }        }      },      "sequence_number": "0",      "type": "0x1::coin::DepositEvent",      "data": {        "amount": "1000"      }    }  ],  "timestamp": "1660615531147935",  "type": "user_transaction"}

There's a lot of information in a transaction:

• version indicates the globally unique identifier for this transaction, its ordered position in all the committed transactions on the Blockchain
• sender is the account address of the entity that submitted the transaction
• gas_used is the units paid for executing the transaction
• success and vm_status indicate whether or not the transaction successfully executed and any reasons why it might not have
• changes include the final values for any state resources that have been modified during the execution of the transaction
• events contain all the events emitted during the transaction execution
• timetstamp is the near real time timestamp of the transactions execution

If success is false, then vm_status will contain an error code or message that resulted in the transaction failing to succeed. When success is false, changes will be limited to gas deducted from the account and the sequence number incrementing. There will be no events.

Each event in events is differentiated by an key. The key is derived from the guid from changes. Specifically, the key is a 40-byte hex string where the first 8-bytes (or 16 characters) are the little endian representation of the creation_num in the guid of the changes event and the remaining characters are the account address. As events do not dictate what emitted them, it is imperative to track the path in changes to determine the source of an event. In particular, each CoinStore<T> has both a WithdrawEvent and a DepositEvent, based upon the type of coin. In order to determine which coin based upon a transaction, an indexer can compare the guid::creation_num in a changes event combined with the address to the key for events in events.

Using the above example, events[1].guid is equivalent to changes[0].data.data.deposit_events.guid, which is {"addr": "0x5098df8e7969b58ab3bd2d440c6203f64c60a1fd5c08b9d4abe6ae4216246c3e", "creation_num": "2"}.

tip

The key field will be going away in favor of guid

### Querying events​

Aptos provides clear and canonical events for all withdraw and deposit of coins. This can be used in coordination with the associated transactions to present to a user the change of their account balance over time, when that happened, and what caused it. With some amount of additional parsing, metadata such as the transaction type and the other parties involved can also be shared.

Events can be queried by the events by handle url: https://{rest_api_server}/accounts/{address}/events/0x1::coin::CoinStore<0x1::aptos_coin::AptosCoin>/withdraw_events

[  {    "version":"13629679",    "key": "0x0300000000000000cb2f940705c44ba110cd3b4f6540c96f2634938bd5f2aabd6946abf12ed88457",    "guid": {      "id": {        "addr": "0x810026ca8291dd88b5b30a1d3ca2edd683d33d06c4a7f7c451d96f6d47bc5e8b",        "creation_num": "3"        }      }    },    "sequence_number": "0",    "type": "0x1::coin::WithdrawEvent",    "data": {      "amount": "1000"    }  }]

More information can be gathered from the transaction that generated the event. To do so, query: https://{rest_server_api}/transactions/by_version/{version}, where {version} is the same value as the {version} in the event query.

tip

When tracking full movement of coins, normally events are sufficient. 0x1::aptos_coin::AptosCoin, however, requires considering gas_used for each transaction sent from the given account. To reduce unnecessary overheads, extracting gas fees due to transactions does not emit an event. All transactions for an account can be retrieved from this API: https://{rest_server_api}/accounts/{address}/transactions.

### Tracking coin balance changes​

Consider the transaction from the earlier section, but now with an arbitrary coin 0x1337::my_coin::MyCoin and some gas parameters changed:

{  "version": "13629679",  "gas_used": "20",  "success": true,  "vm_status": "Executed successfully",  "changes": [    {      "address": "0xb258b91eee04111039320a85b0c24a2dd433909e14a6b5c32ee722e0fdecfddc",      "data": {        "type": "0x1::coin::CoinStore<0x1337::my_coin::MyCoin>",        "data": {          "coin": {            "value": "1000"          },          "deposit_events": {            "counter": "1",            "guid": {              "id": {                "addr": "0x5098df8e7969b58ab3bd2d440c6203f64c60a1fd5c08b9d4abe6ae4216246c3e",                "creaton_num": "2",              }            }          },          ...        }      },      "type": "write_resource"    },    ...  ],  "sender": "0x810026ca8291dd88b5b30a1d3ca2edd683d33d06c4a7f7c451d96f6d47bc5e8b",  "sequence_number": "0",  "max_gas_amount": "2000",  "gas_unit_price": "110",  "expiration_timestamp_secs": "1660616127",  "payload": {    "function": "0x1::coin::transfer",    "type_arguments": [      "0x1337::my_coin::MyCoin"    ],    "arguments": [      "0x5098df8e7969b58ab3bd2d440c6203f64c60a1fd5c08b9d4abe6ae4216246c3e",      "1000"    ],    "type": "entry_function_payload"  },  "events": [    {      "key": "0x0300000000000000810026ca8291dd88b5b30a1d3ca2edd683d33d06c4a7f7c451d96f6d47bc5e8b",      "guid": {        "id": {          "addr": "0x810026ca8291dd88b5b30a1d3ca2edd683d33d06c4a7f7c451d96f6d47bc5e8b",          "creation_num": "3"          }        }      },      "sequence_number": "0",      "type": "0x1::coin::WithdrawEvent",      "data": {        "amount": "1000"      }    },    {      "key": "0x02000000000000005098df8e7969b58ab3bd2d440c6203f64c60a1fd5c08b9d4abe6ae4216246c3e",      guid": {        "id": {          "addr": "0x5098df8e7969b58ab3bd2d440c6203f64c60a1fd5c08b9d4abe6ae4216246c3e",          "creation_num": "2"          }        }      },      "sequence_number": "0",      "type": "0x1::coin::DepositEvent",      "data": {        "amount": "1000"      }    }  ],  "timestamp": "1660615531147935",  "type": "user_transaction"}

There are three balance changes in this transaction:

1. A withdrawal of 1000 of 0x1337::my_coin::MyCoin from the sending account 0x810026ca8291dd88b5b30a1d3ca2edd683d33d06c4a7f7c451d96f6d47bc5e8b
2. A deposit of 1000 of 0x1337::my_coin::MyCoin to receiving account 0x5098df8e7969b58ab3bd2d440c6203f64c60a1fd5c08b9d4abe6ae4216246c3e
3. A gas fee 2200 of 0x1::aptos_coin::AptosCoin from the transaction sending account 0x810026ca8291dd88b5b30a1d3ca2edd683d33d06c4a7f7c451d96f6d47bc5e8b

To retrieve the withdrawal information you can take these steps:

1. Scan the changes for 0x1::coin::CoinStore<CoinType>. Note the CoinType is a generic signifying which coin is stored in the store. In this example, the CoinType is 0x1337::my_coin::MyCoin.
2. Retrieve both the guid for withdraw_events. In this example, the guid contains addr 0x810026ca8291dd88b5b30a1d3ca2edd683d33d06c4a7f7c451d96f6d47bc5e8b and creation_num 3.
3. Scan for events with this guid, and extract the event associated with it. In this example, it is the 0x1::coin::WithdrawEvent.
4. The amount field will be the number of CoinType removed from the account in the guid. In this example, it is 1000.

To retrieve the deposit information, it's the same as withdrawal except:

1. The guid used is under deposit_events
2. The amount will be a positive increase on the account's balance.
3. The event's name will be 0x1::coin::DepositEvent

To retrieve the gas fee:

1. The gas_used field must be multiplied times the gas_unit_price. In this example, gas_used=20 and gas_unit_price=110 so the total gas coins withdrawn is 2200.
2. Gas is always 0x1::aptos_coin::AptosCoin

To retrieve information about the number of decimals of the coin:

1. You can retrieve the number of decimals for a coin via it's 0x1::coin::CoinInfo<CoinType>.
2. This will be located at the address of the coin type. In this example, you would need to lookup 0x1::coin::CoinInfo<0x1337::my_coin::MyCoin> at address 0x1337.
tip

If you always use the events in this manner, you won't miss any balance changes for an account. By monitoring the events, it will include any balance changes in the 0x1::coin::CoinStore:

1. Coin mints
2. Coin burns
3. Coin transfers
4. Staking coins
5. Withdrawing staked coins
6. Transfers not derived from coin::transfer

To create some sample data to explore see "Your first transaction".