826d66bb56
Currently, block templates that provide work to PoW miners are generated
in response to them calling the getwork RPC. However, unlike in a pure
PoW system, in Decred, when a new block is connected, a new block
template extending that block can't be generated until the minimum
number of required votes for it has been received. This poses a
challenge for PoW miners because they are typically unaware of when
votes are received. Consequently, miners poll getwork waiting until
they see a new template generated when a new block is connected.
Since only a minimum of 3 votes is required to build a template and new
templates are only generated in response to the miners calling getwork,
as just described, miners often, unfortunately, end up receiving a
template that only has 3 votes and begin working on it. Worse, since
many new miners are not well versed in the intricacies of Decred voting,
they often aren't aware they really need to continue polling for new
votes and immediately switch over to the new template with more votes in
it in order to avoid receiving a reduced subsidy. This often results in
new miners producing blocks with 3 votes.
Moreover, blocks with less than all 5 votes are not only undesirable for
the PoW miners because they receive reduced subsidy, they're also
undesirable for PoS stakers since it means the votes that were not
included don't receive the reward despite having voted simply due to a
propagation delay.
Another issue with the existing approach is that since it takes a bit of
time to generate and validate templates, particularly as the number of
transactions they include rises, miners periodically requesting updated
work to include the latest transactions have to wait for the template to
be built when they call getwork once the existing cached template is
expired. This can result in undesirable delays for them.
In order to address the need to wait for templates to be built in that
case, there was recently some preliminary work that implemented a
background template generator which asynchronously generates templates
in response to events with the intention of allowing access to the
latest one without said random delays in the future.
However, that preliminary work did not address the issues around vote
propagation such as the tendency for miners to end up with the first
block template only containing the 3 fastest-propagating votes, nor did
it have robust handling for all potential corner cases and error
conditions.
Consequently, this overhauls the background block template generator to
add support for intelligent vote propagation handling, handle chain
reorganization to alternative blocks in the case the current tip is
unable to obtain enough votes, provide a subscription for a stream of
template updates, handle template generation errors, consider the
synchronization state as a part of determining if the chain is current,
track the reason for template updates, block current template retrieval
during operations that are in the process of making it stale, and
correct several corner cases.
It should be noted that this only implements the infrastructure and does
not switch getwork or the CPU miner over to use the background templates
yet. This will be done in future commits.
The following is a high-level overview of the semantics this implements:
- Generate new templates immediately when prior to stake validation
height since no votes are required
- Do not generate templates for intermediate blocks during a chain
reorganization
- Do not generate templates before the chain is considered synchronized
- Prefer to create templates with maximum votes through the use of a
timeout once the minimum votes have been received to provide the votes
an opportunity to propagate with a fallback for immediate generation
as soon as all votes are received
- In the case the timeout expires and a template is created with less
than the maximum number of votes, generate a new template
immediately upon receiving more votes for the block it extends
- In the event there are competing blocks at the current tip, prefer to
build a template on the first seen block so long as it receives the
minimum number of required votes within a few seconds
- Generate new templates immediately when a block is disconnected to
support future planned chain invalidation capabilities
- Generate new templates periodically when there are new regular transactions
to include
- Schedule retries in the rare event template generation fails
- Allow clients to subscribe for updates every time a new template is
successfully generated along with a reason why it was generated
- Provide direct access to the most-recently generated template
- Block while generating new templates that will make the current template
stale (e.g. new parent or new votes)
The following is a high-level overview of the changes:
- Initialize PRNG once instead of at every invocation
- Implement an asynchronous queue for all events to ensure normal chain
and vote processing is not blocked
- Provide ability to subscribe for all template updates over a single
channel to ensure none can indadvertently be missed as is possible with
the current design
- Always deliver existing template upon registration
- Drop templates if the receiver falls too far behind in the same way
tickers work
- Introduce tracking the reason for template updates so that it can
ultimately be presented to miners to allow them to make better
decisions about when to force their workers to switch
- Consider the current network sync state when checking if the chain is
current
- Introduce a regen handler state such as timeouts, blocks to monitor
for votes, and the block the next template should extend
- Add logic for selectively reacting to votes on the block the current
template is extending, the current tip, and alternate competing blocks
at the current tip using the aforementioned semantics
- Perform all template generation in separate goroutines with a
cancellable context
- Cancel any in progress templates that are being generated whenever
generating a new one
- Introduce blocking current template retrieval when a new template that
would cause the existing one to become stale is being generated
- Modify periodic regen handling to use a resettable timer for better
efficiency and more fine grained control
- Remove template pool as that is something that should be handled by
the code that is actually handing the templates out
- Rename and export the event notification funcs to make it clear
they're not internal functions and also make it easier to eventually
move the code into the mining package
- Expand and improve comments
- Store and return template generation errors
- Schedule retry in the case of failure
- Correct several cases that were not being handled correctly and some
undesirable behaviors such as block disconnects (as opposed to
reorgs), reorganization to side chains, and notifying with stale
templates
|
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|---|---|---|
| .. | ||
| doc.go | ||
| error.go | ||
| go.mod | ||
| go.sum | ||
| log.go | ||
| mempool_test.go | ||
| mempool.go | ||
| policy_test.go | ||
| policy.go | ||
| README.md | ||
mempool
Package mempool provides a policy-enforced pool of unmined Decred transactions.
A key responsibility of the Decred network is mining transactions – regular transactions and stake transactions – into blocks. In order to facilitate this, the mining process relies on having a readily-available source of transactions to include in a block that is being solved.
At a high level, this package satisfies that requirement by providing an in-memory pool of fully validated transactions that can also optionally be further filtered based upon a configurable policy.
The Policy configuration options has flags that control whether or not "standard" transactions and old votes are accepted into the mempool. In essence, a "standard" transaction is one that satisfies a fairly strict set of requirements that are largely intended to help provide fair use of the system to all users. It is important to note that what is considered to be a "standard" transaction changes over time as policy and consensus rules evolve. For some insight, at the time of this writing, an example of some of the criteria that are required for a transaction to be considered standard are that it is of the most-recently supported version, finalized, does not exceed a specific size, and only consists of specific script forms.
Since this package does not deal with other Decred specifics such as network communication and transaction relay, it returns a list of transactions that were accepted which gives the caller a high level of flexibility in how they want to proceed. Typically, this will involve things such as relaying the transactions to other peers on the network and notifying the mining process that new transactions are available.
This package has intentionally been designed so it can be used as a standalone package for any projects needing the ability create an in-memory pool of Decred transactions that are not only valid by consensus rules, but also adhere to a configurable policy
Feature Overview
The following is a quick overview of the major features. It is not intended to be an exhaustive list.
- Maintain a pool of fully validated transactions
- Reject non-fully-spent duplicate transactions
- Reject coinbase transactions
- Reject double spends (both from the chain and other transactions in pool)
- Reject invalid transactions according to the network consensus rules
- Full script execution and validation with signature cache support
- Individual transaction query support
- Stake transaction support (ticket purchases, votes and revocations)
- Option to accept or reject old votes
- Orphan transaction support (transactions that spend from unknown outputs)
- Configurable limits (see transaction acceptance policy)
- Automatic addition of orphan transactions that are no longer orphans as new transactions are added to the pool
- Individual orphan transaction query support
- Configurable transaction acceptance policy
- Option to accept or reject standard transactions
- Option to accept or reject transactions based on priority calculations
- Rate limiting of low-fee and free transactions
- Non-zero fee threshold
- Max signature operations per transaction
- Max orphan transaction size
- Max number of orphan transactions allowed
- Additional metadata tracking for each transaction
- Timestamp when the transaction was added to the pool
- Most recent block height when the transaction was added to the pool
- The fee the transaction pays
- The starting priority for the transaction
- Manual control of transaction removal
- Recursive removal of all dependent transactions
Installation and Updating
$ go get -u github.com/decred/dcrd/mempool
License
Package mempool is licensed under the copyfree ISC License.