user66526: I have been reading a lot about the validation of transactions. I understand the actual method in which a transaction is composed (inputs and outputs). However, I don't know what happens with that transaction once its "ready":

Do miners check it before including it in a block?
Do other nodes check it and where there is a certain consensuous miners include it into a block? Who composes the blocks then? Are all miners trying to solve the same block at the same time or different blocks?

Thanks in advance

user85: 
Do miners check it before including it in a block?

Yes. If they include an invalid transaction in a block, that block will be invalid.

Do other nodes check it and where there is a certain consensuous miners include it into a block?

Miners include it in a block if they wish to and if it's valid. They don't have to care about what anyone else thinks. They can put whatever valid transactions in their blocks that they wish to. Typically, they'll pick transactions that maximize the fees they make.

Who composes the blocks then?

Every miner composes blocks and attempts to mine the blocks the compose.

Are all miners trying to solve the same block at the same time or different blocks?

That's a matter of definition. Each miner is trying to produce a valid next block for the longest chain of valid blocks that they know about. But each miner is trying to mine a block that includes a transaction that pays them the block reward. So in that sense, they're trying to mine different blocks.

user38618: 
Other nodes check if the transaction is valid, and if it is, they relay the transaction to other nodes. Mining pools have also (at least one) nodes, they verify it too, and if it's valid and the fee is enough, they put it in the block they're working on.
Do miners check it before including it in a block?

Yep. If they include an invalid transaction, the other nodes will not accept that block, and that miner won't earn the block reward for that block. (currently 125k dollars!)

Do other nodes check it and where there is a certain consensuous miners include it into a block

Yes.

Who composes the blocks then?

Miners.

Are all miners trying to solve the same block at the same time or different blocks?

They are trying to find a block with the same height, but with different transactions, different nonces (a random number - related to block hash and difficulty) and different block reward addresses.

66526: <p>I have been reading a lot about the validation of transactions. I understand the actual method in which a transaction is composed (inputs and outputs). However, I don't know what happens with that transaction once its "ready":</p>

<ol>
<li>Do miners check it before including it in a block?</li>
<li>Do other nodes check it and where there is a certain consensuous miners include it into a block? Who composes the blocks then? Are all miners trying to solve the same block at the same time or different blocks?</li>
</ol>

<p>Thanks in advance</p>
 85: <blockquote>
  <p>Do miners check it before including it in a block?</p>
</blockquote>

<p>Yes. If they include an invalid transaction in a block, that block will be invalid.</p>

<blockquote>
  <p>Do other nodes check it and where there is a certain consensuous miners include it into a block?</p>
</blockquote>

<p>Miners include it in a block if they wish to and if it's valid. They don't have to care about what anyone else thinks. They can put whatever valid transactions in their blocks that they wish to. Typically, they'll pick transactions that maximize the fees they make.</p>

<blockquote>
  <p>Who composes the blocks then?</p>
</blockquote>

<p>Every miner composes blocks and attempts to mine the blocks the compose.</p>

<blockquote>
  <p>Are all miners trying to solve the same block at the same time or different blocks?</p>
</blockquote>

<p>That's a matter of definition. Each miner is trying to produce a valid next block for the longest chain of valid blocks that they know about. But each miner is trying to mine a block that includes a transaction that pays them the block reward. So in that sense, they're trying to mine different blocks.</p>
 38618: <blockquote>
<p>Other nodes check if the transaction is valid, and if it is, they relay the transaction to other nodes. Mining pools have also (at least one) nodes, they verify it too, and if it's valid and the fee is enough, they put it in the block they're working on.</p>
<p>Do miners check it before including it in a block?</p>
</blockquote>
<p>Yep. If they include an invalid transaction, the other nodes will not accept that block, and that miner won't earn the block reward for that block. (currently 125k dollars!)</p>
<blockquote>
<p>Do other nodes check it and where there is a certain consensuous miners include it into a block</p>
</blockquote>
<p>Yes.</p>
<blockquote>
<p>Who composes the blocks then?</p>
</blockquote>
<p>Miners.</p>
<blockquote>
<p>Are all miners trying to solve the same block at the same time or different blocks?</p>
</blockquote>
<p>They are trying to find a block with the same height, but with different transactions, different nonces (a random number - related to block hash and difficulty) and different block reward addresses.</p>
