High Availability

To enable high availability to the RPC Pub/Sub API over WebSockets or the JSON-RPC API run and synchronize multiple Pantheon nodes to the network. Use a load balancer to distribute requests across nodes in the cluster that are ready to receive requests.

Determining When a Node is Ready

Use the readiness endpoint to determine when a node is ready.

!!! note The minimum number of peers and number of blocks from the best known block for a node to be considered ready is deployment specific.

Transaction Nonces

The account nonce for the next transaction is obtained using eth_getTransactionCount. The account nonce depends on the transactions in the transaction pool. If eth_getTransactionCount and eth_sendRawTransaction requests for a specific account are sent to multiple nodes, the eth_getTransactionCount results can be incorrect.

!!! note If using private transactions, eea_getTransactionCount is used to obtain the account nonce and eea_sendRawTransaction to send private transactions.

To get correct account nonces when distributing requests across a cluster, do one of:

Track the next nonce outside of the Pantheon node (as MetaMask does)
Configure the load balancer in sticky mode so requests from a specific account are sent to a single node unless that node is unavailable.

Subscriptions

You can subscribe to events using:

We recommend using RPC Pub/Sub over WebSockets because WebSockets connections are associated a specific node and do not require using the load balancer in sticky mode.

If using filters over HTTP, configure the load balancer in sticky mode to associate the subscription with a specific node.

Recovering from Dropped Subscriptions

Subscriptions can be dropped if:

WebSockets connection is disconnected
Node serving the subscription is removed from the ready pool

If a subscription is dropped, events can be missed while reconnecting to a different node. To recover dropped messages, create another subscription and follow the process for that subscription type:

newHeads
logs
newPendingTransactions
droppedPendingTransactions
syncing

New Headers

Use eth_getBlockByNumber to request information on blocks from the last block before the subscription dropped to the first block received from the new subscription.

Logs

Use eth_getLogs to request logs from the block number of the last log received before the subscription dropped to the current chain head.

New Pending Transactions

Use txpool_pantheonTransactions to request all pending transactions for the new node.

!!! note Nodes do not all store the same pending transactions.

Dropped Pending Transactions