/**
* Given a node which was first seen at ``first_seen'' and last seen at
* ``last_seen'', return probability that node still be alive now.
*
* @param first_seen first time node was seen / created
* @param last_seen last time node was seen
*
* @return the probability that the node be still alive now.
*/
double
stable_still_alive_probability(time_t first_seen, time_t last_seen)
{
time_delta_t life;
time_delta_t elapsed;
life = delta_time(last_seen, first_seen);
if (life <= 0)
return 0.0;
elapsed = delta_time(tm_time(), last_seen);
return stable_alive_probability(life, elapsed);
}
/**
* Estimate probability of presence for a value published to some roots in
* a given time frame.
*
* @param d how many seconds in the future?
* @param rs the STORE lookup path, giving root candidates
* @param status the array of STORE status for each entry in the path
*
* @return an estimated probability of presence of the value in the network.
*/
double
stable_store_presence(time_delta_t d,
const lookup_rs_t *rs, const guint16 *status)
{
double q = 1.0;
size_t i;
size_t count = lookup_result_path_length(rs);
/*
* We may be called by publish callbacks invoked to clean up because
* the operation was cancelled. Maybe the DHT was disabled during the
* operation, meaning our data structures have been cleaned up? In that
* case, abort immediately.
*
* NOTE: this is not an assertion, it can happen in practice and needs to
* be explicitly checked for.
*/
if (NULL == db_lifedata) /* DHT disabled dynamically */
return 0.0;
/*
* The probability of presence is (1 - q) where q is the probability
* that the value be lost by all the nodes, i.e. that all the nodes
* to which the value was published to be gone in "d" seconds.
*/
for (i = 0; i < count; i++) {
if (status[i] == STORE_SC_OK) {
const knode_t *kn = lookup_result_nth_node(rs, i);
struct lifedata *ld = get_lifedata(kn->id);
if (NULL == ld) {
return 0.0; /* Cannot compute a suitable probability */
} else {
time_delta_t alive = delta_time(ld->last_seen, ld->first_seen);
double p = stable_alive_probability(alive, d);
q *= (1.0 - p); /* (1 - p) is proba this node will be gone */
}
}
}
return 1.0 - q;
}
/**
* Given a node which was first seen at ``first_seen'' and last seen at
* ``last_seen'', return probability that node still be alive now.
*
* @param first_seen first time node was seen / created
* @param last_seen last time node was seen
*
* @return the probability that the node be still alive now.
*/
double
stable_still_alive_probability(time_t first_seen, time_t last_seen)
{
time_delta_t life;
time_delta_t elapsed;
life = delta_time(last_seen, first_seen);
if (life <= 0)
return 0.0;
elapsed = delta_time(tm_time(), last_seen);
/*
* Safety precaution: regardless of the past lifetime of the node, if
* we have not heard from it for more than STABLE_UPPER_THRESH, then
* consider it dead.
*/
return elapsed < STABLE_UPPER_THRESH ?
stable_alive_probability(life, elapsed) : 0.0;
}
