user60462: How do I get number of pending transactions (queued and waiting for confirmation) for an arbitrary blockchain? Do I need to download that blockchain? There should be a more adequate way.

user24974: Due to the decentralized nature of blockchains, the state of transactions is relative.  A pending transaction is one that your node has seen and verified as valid, yet your node has not seen a block that includes it.  This might not be the same state that other nodes see.
For example, one-block chain forks are a common occurrence, where there is a disagreement on the network as to which block is the next valid block.  This will usually resolve itself when a block is found on top of one of these two blocks, but in the meantime, there is disagreement about which transactions have been included in a block.
There is also the possibility that your node has not seen a transaction, whereas other nodes (including mining nodes) have.  You wouldn't have this transaction in your list of pending transactions, even though there are miners actively attempting to include it in a block.  The opposite is also true, where you have seen a transaction, but miners are ignoring it for whatever reason.  Bitcoin is actually pretty good about these scenarios, so it normally isn't an issue, but it is a possibility.
In addition, there is the question of a transaction's validity.  Even though you can be fairly certain that a transaction is valid without independently verifying it with a downloaded copy of the blockchain, you can only be completely sure if you do independently verify it.  If your node is ever wrong or in disagreement with the network about a transaction's validity, then you may consider it pending whereas mining nodes will never include it in a block.  Again, the opposite is also possible.
In order to see which transactions your node (assuming Bitcoin or Bitcoin-based) has "pending", you can use the getrawmempool RPC command.
Further reading:
bitcoind getrawmempool without downloading the full blockchain
Is it possible to view unconfirmed transactions with the bitcoin client?

60462: <p>How do I get number of pending transactions (queued and waiting for confirmation) for an arbitrary blockchain? Do I need to download that blockchain? There should be a more adequate way.</p>
 24974: <p>Due to the decentralized nature of blockchains, the state of transactions is relative.  A pending transaction is one that your node has seen and verified as valid, yet your node has <em>not</em> seen a block that includes it.  This might not be the same state that other nodes see.</p>

<p>For example, one-block chain forks are a common occurrence, where there is a disagreement on the network as to which block is the next valid block.  This will usually resolve itself when a block is found on top of one of these two blocks, but in the meantime, there is disagreement about which transactions have been included in a block.</p>

<p>There is also the possibility that your node has not seen a transaction, whereas other nodes (including mining nodes) have.  You wouldn't have this transaction in your list of pending transactions, even though there are miners actively attempting to include it in a block.  The opposite is also true, where you have seen a transaction, but miners are ignoring it for whatever reason.  Bitcoin is actually pretty good about these scenarios, so it normally isn't an issue, but it is a possibility.</p>

<p>In addition, there is the question of a transaction's validity.  Even though you can be fairly certain that a transaction is valid without independently verifying it with a downloaded copy of the blockchain, you can only be completely sure if you <em>do</em> independently verify it.  If your node is ever wrong or in disagreement with the network about a transaction's validity, then you may consider it pending whereas mining nodes will never include it in a block.  Again, the opposite is also possible.</p>

<p>In order to see which transactions your node (assuming Bitcoin or Bitcoin-based) has "pending", you can use the <code>getrawmempool</code> RPC command.</p>

<p>Further reading:</p>

<p><a href="https://bitcoin.stackexchange.com/questions/35811/bitcoind-getrawmempool-without-downloading-the-full-blockchain">bitcoind getrawmempool without downloading the full blockchain</a></p>

<p><a href="https://bitcoin.stackexchange.com/questions/20720/is-it-possible-to-view-unconfirmed-transactions-with-the-bitcoin-client">Is it possible to view unconfirmed transactions with the bitcoin client?</a></p>
