user5157: I am wondering if I can use node.js and levelup to access a copy of the blockchain database directly.
But from what I can see, I need to know the name of the key(s) before I can get any data, as this is used in the get method of levelup.
However, I have not been able to find out anything about the possible key(s) associated with the values in the key value pairs, so I'm at a loss as to how I might retrieve the data.
Firstly, what are the keys in the key value pairs, and secondly is it possible just to select the first n records?

user99969: This post is pretty old and is really a reference to many, as it's been to me, but it took me quite a while to grasp it totally.
So I wrote this blog post to clarify certain points, particularly the key parameter endianness: https://imil.net/blog/posts/2020/bitcoin-leveldb-debugging/
One specific point is that the accepted reply states that the unspent transaction output record for that transaction is a format of c+32 bytes transaction hash, when it is really C (capital C, i.e. 0x43) instead.

user133739: I have been googling around a bit and keep seeing the following:
'b' + 32-byte block hash -> block index record. Each record stores:
The block header.
The height.
The number of transactions.
To what extent this block is validated.
In which file, and where in that file, the block data is stored.
In which file, and where in that file, the undo data is stored
This is fantastic...thanks..but...
What is the structure of this data?? How many bytes is 'block header'? How many bytes is 'the height', etc. Are these 32 bit integers? 64 bit? Are they big-endian, little-endian?
What would it look like, say, as a C style struct using stdint formats?

user5157: OK I know I shouldn't really answer my own question but... in the absence of a response to this question, I did a bit of hunting.
Github provided the answer in a file found in the bitcoin-leveldb repository. 
The path to a text file containing the information leveldb->doc->table_format.txt
In short, there is no easily describable table like structure. There are several, shall I call them "nested" structures in the database including the fact that the database physically stores the data in seperate logical files. 
Here is the table_format.txt file contents as of this post. 
File format
  <beginning_of_file>
  [data block 1]
  [data block 2]
  ...
  [data block N]
  [meta block 1]
  ...
  [meta block K]
  [metaindex block]
  [index block]
  [Footer]        (fixed size; starts at file_size - sizeof(Footer))
  <end_of_file>

The file contains internal pointers.  Each such pointer is called
a BlockHandle and contains the following information:
  offset:       varint64
  size:         varint64
See https://developers.google.com/protocol-buffers/docs/encoding#varints
for an explanation of varint64 format.

(1) The sequence of key/value pairs in the file are stored in sorted
order and partitioned into a sequence of data blocks.  These blocks
come one after another at the beginning of the file.  Each data block
is formatted according to the code in block_builder.cc, and then
optionally compressed.

(2) After the data blocks we store a bunch of meta blocks.  The
supported meta block types are described below.  More meta block types
may be added in the future.  Each meta block is again formatted using
block_builder.cc and then optionally compressed.

(3) A "metaindex" block.  It contains one entry for every other meta
block where the key is the name of the meta block and the value is a
BlockHandle pointing to that meta block.

(4) An "index" block.  This block contains one entry per data block,
where the key is a string >= last key in that data block and before
the first key in the successive data block.  The value is the
BlockHandle for the data block.

(6) At the very end of the file is a fixed length footer that contains
the BlockHandle of the metaindex and index blocks as well as a magic number.
       metaindex_handle: char[p];    // Block handle for metaindex
       index_handle:     char[q];    // Block handle for index
       padding:          char[40-p-q]; // zeroed bytes to make fixed length
                                       // (40==2*BlockHandle::kMaxEncodedLength)
       magic:            fixed64;    // == 0xdb4775248b80fb57 (little-endian)

"filter" Meta Block
If a "FilterPolicy" was specified when the database was opened, a
filter block is stored in each table.  The "metaindex" block contains
an entry that maps from "filter.<N>" to the BlockHandle for the filter
block where "<N>" is the string returned by the filter policy's
"Name()" method.

The filter block stores a sequence of filters, where filter i contains
the output of FilterPolicy::CreateFilter() on all keys that are stored
in a block whose file offset falls within the range

    [ i*base ... (i+1)*base-1 ]

Currently, "base" is 2KB.  So for example, if blocks X and Y start in
the range [ 0KB .. 2KB-1 ], all of the keys in X and Y will be
converted to a filter by calling FilterPolicy::CreateFilter(), and the
resulting filter will be stored as the first filter in the filter
block.

The filter block is formatted as follows:

     [filter 0]
     [filter 1]
     [filter 2]
     ...
     [filter N-1]

     [offset of filter 0]                  : 4 bytes
     [offset of filter 1]                  : 4 bytes
     [offset of filter 2]                  : 4 bytes
     ...
     [offset of filter N-1]                : 4 bytes

     [offset of beginning of offset array] : 4 bytes
     lg(base)                              : 1 byte

The offset array at the end of the filter block allows efficient
mapping from a data block offset to the corresponding filter.

"stats" Meta Block
This meta block contains a bunch of stats.  The key is the name
of the statistic.  The value contains the statistic.
TODO(postrelease): record following stats.
  data size
  index size
  key size (uncompressed)
  value size (uncompressed)
  number of entries
  number of data blocks


user208: Bitcoind since 0.8 maintains two databases, the block index (in $DATADIR/blocks/index) and the chainstate (in $DATADIR/chainstate). The block index maintains information for every block, and where it is stored on disk. The chain state maintains information about the resulting state of validation as a result of the currently best known chain.
Inside the block index, the used key/value pairs are:

'b' + 32-byte block hash -> block index record. Each record stores:

The block header.
The height.
The number of transactions.
To what extent this block is validated.
In which file, and where in that file, the block data is stored.
In which file, and where in that file, the undo data is stored.


'f' + 4-byte file number -> file information record. Each record stores:

The number of blocks stored in the block file with that number.
The size of the block file with that number ($DATADIR/blocks/blkNNNNN.dat).
The size of the undo file with that number ($DATADIR/blocks/revNNNNN.dat).
The lowest and highest height of blocks stored in the block file with that number.
The lowest and highest timestamp of blocks stored in the block file with that number.


'l' -> 4-byte file number: the last block file number used.
'R' -> 1-byte boolean ('1' if true): whether we're in the process of reindexing.
'F' + 1-byte flag name length + flag name string -> 1 byte boolean ('1' if true, '0' if false): various flags that can be on or off. Currently defined flags include:

'txindex': Whether the transaction index is enabled.


't' + 32-byte transaction hash -> transaction index record. These are optional and only exist if 'txindex' is enabled (see above). Each record stores:

Which block file number the transaction is stored in.
Which offset into that file the block the transaction is part of is stored at.
The offset from the start of that block to the position where that transaction itself is stored.



Inside the chain state database, the following key/value pairs are stored:

'C'+ 32-byte transaction hash + output index length + output index (v0.15 onwards) -> A single unspent transaction output (UTXO) record. Each record contains information about the UTXO at the specified output index of the given transaction. This information consists of:

Whether the transaction was a coinbase or not.
Which height block contains the transaction.
The scriptPubKey and amount for this unspent output.


'c' + 32-byte transaction hash (pre-v0.14) -> unspent transaction output record for that transaction. Unlike 'C', this entry represents all UTXOs from a single transaction. These records are only present for transactions that have at least one unspent output left. Each record stores:

The version of the transaction.
Whether the transaction was a coinbase or not.
Which height block contains the transaction.
Which outputs of that transaction are unspent.
The scriptPubKey and amount for those unspent outputs.


'B' -> 32-byte block hash: the block hash up to which the database represents the unspent transaction outputs.

Latest version of bitcoind(please add version compatibility) uses obfuscation of the value in key/value pair . So you need to XOR with the obfuscation key to get the real value.
I won't go into the specific serialization details of the particular records. They're often specially designed to be compact on disk, and not really intended to be easily usable by other applications (LevelDB doesn't support concurrent access from multiple applications anyway). There are several RPC methods for querying data from the databases (getblock, gettxoutsetinfo, gettxout) without needing direct access.
As you can see, only headers are stored inside this database. The actual blocks and transactions are stored in the block files, which are not databases, but just raw append-only files that contain the blocks in network format.
As to your second question: what is n? If you just want to access some records, sure, iterate over the keys and stop when you've read enough.

5157: <p>I am wondering if I can use node.js and levelup to access a copy of the blockchain database directly.</p>

<p>But from what I can see, I need to know the name of the key(s) before I can get any data, as this is used in the <code>get</code> method of levelup.</p>

<p>However, I have not been able to find out anything about the possible key(s) associated with the values in the key value pairs, so I'm at a loss as to how I might retrieve the data.</p>

<p>Firstly, what are the keys in the key value pairs, and secondly is it possible just to select the first <code>n</code> records?</p>
 99969: <p>This post is pretty old and is really a reference to many, as it's been to me, but it took me quite a while to grasp it totally.<br>
So I wrote this blog post to clarify certain points, particularly the key parameter endianness: <a href="https://imil.net/blog/posts/2020/bitcoin-leveldb-debugging/" rel="nofollow noreferrer">https://imil.net/blog/posts/2020/bitcoin-leveldb-debugging/</a></p>

<p>One specific point is that the accepted reply states that the <em>unspent transaction output record for that transaction</em> is a format of <code>c</code>+32 bytes transaction hash, when it is really <code>C</code> (capital C, i.e. <code>0x43</code>) instead.</p>
 133739: <p>I have been googling around a bit and keep seeing the following:</p>
<p>'b' + 32-byte block hash -&gt; block index record. Each record stores:
The block header.
The height.
The number of transactions.
To what extent this block is validated.
In which file, and where in that file, the block data is stored.
In which file, and where in that file, the undo data is stored</p>
<p>This is fantastic...thanks..but...</p>
<p>What is the structure of this data?? How many bytes is 'block header'? How many bytes is 'the height', etc. Are these 32 bit integers? 64 bit? Are they big-endian, little-endian?</p>
<p>What would it look like, say, as a C style struct using stdint formats?</p>
 5157: <p>OK I know I shouldn't really answer my own question but... in the absence of a response to this question, I did a bit of hunting.</p>

<p>Github provided the answer in a file found in the <a href="https://github.com/bitcoin/leveldb" rel="noreferrer">bitcoin-leveldb</a> repository. </p>

<p>The path to a text file containing the information <code>leveldb-&gt;doc-&gt;table_format.txt</code></p>

<p>In short, there is no easily describable table like structure. There are several, shall I call them "nested" structures in the database including the fact that the database physically stores the data in seperate logical files. </p>

<p>Here is the <code>table_format.txt</code> file contents as of this post. </p>

<h1>File format</h1>

<pre><code>  &lt;beginning_of_file&gt;
  [data block 1]
  [data block 2]
  ...
  [data block N]
  [meta block 1]
  ...
  [meta block K]
  [metaindex block]
  [index block]
  [Footer]        (fixed size; starts at file_size - sizeof(Footer))
  &lt;end_of_file&gt;

The file contains internal pointers.  Each such pointer is called
a BlockHandle and contains the following information:
  offset:       varint64
  size:         varint64
See https://developers.google.com/protocol-buffers/docs/encoding#varints
for an explanation of varint64 format.

(1) The sequence of key/value pairs in the file are stored in sorted
order and partitioned into a sequence of data blocks.  These blocks
come one after another at the beginning of the file.  Each data block
is formatted according to the code in block_builder.cc, and then
optionally compressed.

(2) After the data blocks we store a bunch of meta blocks.  The
supported meta block types are described below.  More meta block types
may be added in the future.  Each meta block is again formatted using
block_builder.cc and then optionally compressed.

(3) A "metaindex" block.  It contains one entry for every other meta
block where the key is the name of the meta block and the value is a
BlockHandle pointing to that meta block.

(4) An "index" block.  This block contains one entry per data block,
where the key is a string &gt;= last key in that data block and before
the first key in the successive data block.  The value is the
BlockHandle for the data block.

(6) At the very end of the file is a fixed length footer that contains
the BlockHandle of the metaindex and index blocks as well as a magic number.
       metaindex_handle: char[p];    // Block handle for metaindex
       index_handle:     char[q];    // Block handle for index
       padding:          char[40-p-q]; // zeroed bytes to make fixed length
                                       // (40==2*BlockHandle::kMaxEncodedLength)
       magic:            fixed64;    // == 0xdb4775248b80fb57 (little-endian)
</code></pre>

<h2>"filter" Meta Block</h2>

<pre><code>If a "FilterPolicy" was specified when the database was opened, a
filter block is stored in each table.  The "metaindex" block contains
an entry that maps from "filter.&lt;N&gt;" to the BlockHandle for the filter
block where "&lt;N&gt;" is the string returned by the filter policy's
"Name()" method.

The filter block stores a sequence of filters, where filter i contains
the output of FilterPolicy::CreateFilter() on all keys that are stored
in a block whose file offset falls within the range

    [ i*base ... (i+1)*base-1 ]

Currently, "base" is 2KB.  So for example, if blocks X and Y start in
the range [ 0KB .. 2KB-1 ], all of the keys in X and Y will be
converted to a filter by calling FilterPolicy::CreateFilter(), and the
resulting filter will be stored as the first filter in the filter
block.

The filter block is formatted as follows:

     [filter 0]
     [filter 1]
     [filter 2]
     ...
     [filter N-1]

     [offset of filter 0]                  : 4 bytes
     [offset of filter 1]                  : 4 bytes
     [offset of filter 2]                  : 4 bytes
     ...
     [offset of filter N-1]                : 4 bytes

     [offset of beginning of offset array] : 4 bytes
     lg(base)                              : 1 byte

The offset array at the end of the filter block allows efficient
mapping from a data block offset to the corresponding filter.
</code></pre>

<h2>"stats" Meta Block</h2>

<pre><code>This meta block contains a bunch of stats.  The key is the name
of the statistic.  The value contains the statistic.
TODO(postrelease): record following stats.
  data size
  index size
  key size (uncompressed)
  value size (uncompressed)
  number of entries
  number of data blocks
</code></pre>
 208: <p>Bitcoind since 0.8 maintains two databases, the block index (in $DATADIR/blocks/index) and the chainstate (in $DATADIR/chainstate). The block index maintains information for every block, and where it is stored on disk. The chain state maintains information about the resulting state of validation as a result of the currently best known chain.</p>
<p>Inside the block index, the used key/value pairs are:</p>
<ul>
<li><strong>'b' + 32-byte block hash</strong> -&gt; block index record. <a href="https://github.com/bitcoin/bitcoin/blob/f914f1a746d7f91951c1da262a4a749dd3ebfa71/src/chain.h#L95" rel="noreferrer">Each record stores:</a>
<ul>
<li>The block header.</li>
<li>The height.</li>
<li>The number of transactions.</li>
<li>To what extent this block is validated.</li>
<li>In which file, and where in that file, the block data is stored.</li>
<li>In which file, and where in that file, the undo data is stored.</li>
</ul>
</li>
<li><strong>'f' + 4-byte file number</strong> -&gt; file information record. <a href="https://github.com/bitcoin/bitcoin/blob/fcbc8bfa6d10cac4f16699d6e6e68fb6eb98acd0/src/main.h#L392" rel="noreferrer">Each record stores:</a>
<ul>
<li>The number of blocks stored in the block file with that number.</li>
<li>The size of the block file with that number ($DATADIR/blocks/blkNNNNN.dat).</li>
<li>The size of the undo file with that number ($DATADIR/blocks/revNNNNN.dat).</li>
<li>The lowest and highest height of blocks stored in the block file with that number.</li>
<li>The lowest and highest timestamp of blocks stored in the block file with that number.</li>
</ul>
</li>
<li><strong>'l'</strong> -&gt; 4-byte file number: the last block file number used.</li>
<li><strong>'R'</strong> -&gt; 1-byte boolean ('1' if true): whether we're in the process of reindexing.</li>
<li><strong>'F' + 1-byte flag name length + flag name string</strong> -&gt; 1 byte boolean ('1' if true, '0' if false): various flags that can be on or off. Currently defined flags include:
<ul>
<li>'txindex': Whether the transaction index is enabled.</li>
</ul>
</li>
<li><strong>'t' + 32-byte transaction hash</strong> -&gt; transaction index record. These are optional and only exist if 'txindex' is enabled (see above). Each record stores:
<ul>
<li>Which block file number the transaction is stored in.</li>
<li>Which offset into that file the block the transaction is part of is stored at.</li>
<li>The offset from the start of that block to the position where that transaction itself is stored.</li>
</ul>
</li>
</ul>
<p>Inside the chain state database, the following key/value pairs are stored:</p>
<ul>
<li><strong>'C'+ 32-byte transaction hash + output index length + output index</strong> (v0.15 onwards) -&gt; A single unspent transaction output (UTXO) record. Each record contains information about the UTXO at the specified output index of the given transaction. This information consists of:
<ul>
<li>Whether the transaction was a coinbase or not.</li>
<li>Which height block contains the transaction.</li>
<li>The scriptPubKey and amount for this unspent output.</li>
</ul>
</li>
<li><strong>'c' + 32-byte transaction hash</strong> (pre-v0.14) -&gt; unspent transaction output record for that transaction. Unlike 'C', this entry represents all UTXOs from a single transaction. These records are only present for transactions that have at least one unspent output left. <a href="https://github.com/bitcoin/bitcoin/blob/d4a42334d447cad48fb3996cad0fd5c945b75571/src/coins.h#L19" rel="noreferrer">Each record stores:</a>
<ul>
<li>The version of the transaction.</li>
<li>Whether the transaction was a coinbase or not.</li>
<li>Which height block contains the transaction.</li>
<li>Which outputs of that transaction are unspent.</li>
<li>The scriptPubKey and amount for those unspent outputs.</li>
</ul>
</li>
<li><strong>'B'</strong> -&gt; 32-byte block hash: the block hash up to which the database represents the unspent transaction outputs.</li>
</ul>
<p>Latest version of bitcoind(please add version compatibility) uses <a href="https://bitcoin.stackexchange.com/a/52167/26481">obfuscation</a> of the value in key/value pair . So you need to XOR with the obfuscation key to get the real value.</p>
<p>I won't go into the specific serialization details of the particular records. They're often specially designed to be compact on disk, and not really intended to be easily usable by other applications (LevelDB doesn't support concurrent access from multiple applications anyway). There are several RPC methods for querying data from the databases (<code>getblock</code>, <code>gettxoutsetinfo</code>, <code>gettxout</code>) without needing direct access.</p>
<p>As you can see, only headers are stored inside this database. The actual blocks and transactions are stored in the block files, which are not databases, but just raw append-only files that contain the blocks in network format.</p>
<p>As to your second question: what is <code>n</code>? If you just want to access some records, sure, iterate over the keys and stop when you've read enough.</p>
